The 25mm f/4 collects the same amount of light as the 50mm f/4. Yes, the aperture diameter and area are smaller, but the 25mm is collecting light from a wider FOV. So it collects the photons from objects that are outside the FOV for the 50mm. When shooting an evenly illuminated field, both these focal lengths collect the same number of photons per unit of time. And even if we look at a finite sized object, such as Jupiter, the 50/4 will spread the photons over more pixels. The 25/4 will concentrate the photons onto 4x fewer pixels, although it is collecting 4x fewer photons from Jupiter due to smaller aperture size. Therefore, Jupiter’s brightness measured by the camera sensor will have the same brightness for both systems. The 50/4 simply enlarges objects for higher resolution while the 25/4 covers a wider FOV. Both are the same bright. Well, only if their T values are the same, but we are ignoring that in this example. In your example of two scopes, one has a large central obstruction that is not considered in the F value (aperture ratio). When we refer to how “bright” an optic is, we do need to consider this, as well as the transmissivity of the glass elements and reflectivity of any mirrors. Hence the T value.
You're quite right, of course. I should have specified in the video that in the thought experiment with the fictitious 25 mm lenses, they are 25 mm aperture, not focal length.
@@swagonman Let's try another thought experiment: Imagine a lens of 1 mm aperture and 2 mm focal length. It gives us a focal ratio of f/2. Imagine another lens of 100 mm aperture and 200 mm focal length, so it also has a focal ratio of f/2. Our 100 mm aperture lens has 7,854 times the area of the 1 mm aperture lens. While we might say that the tiny lens can make some pixels on a camera sensor bright as quickly as the big lens can make some pixels on a sensor bright, they are not equal. Not by a long shot. In point of fact, the much wider lens moves far more light energy and can affect far more pixels, quickly making a complete image while the 1 mm lens would struggle to illuminate a dot. Thus, we see that the 1 mm lens' f/2 rating is not the same as the 100 mm lens' f/2 rating. None of this even takes into account the greater ability of a wider aperture to resolve detail. Too much for this one rabbit hole. In my posts section, at the suggestion of another person, I did an actual experiment (really a demonstration since this has long since been experimented on) and used a lens of 50 mm aperture and a lens of 70+ mm aperture with the same ISO, shutter speed and and f-stop setting to produce an image of the same object with the same camera body. At equivalent f-stop (focal ratio), the differences in the images are obvious. The bigger lens moves more photons, leading to more information and less noise. Focal ratio did not equally indicate what their performance would be. All things being equal, aperture is what mattered. And that is the point of this video. Focal ratio is not a standard of measure; it is simply a ratio. It is useless as a tool for comparison, except in comparing a lens of a given aperture to itself or other lenses of similar aperture and other characteristics, and even then there are limits.
@@SKYST0RY This is silly. Of course a 2mm f/2 is a different lens than a 200mm f/2. They have vastly different FOV. But they are equally as “fast”. You really ought to take down your video, modify heavily, and repost it. As you wrote it, it is simply incorrect. You should focus on the difference of FOV versus Resolution. Just like terrestrial photography, different focal lengths are needed for different FOV. I have Astrographs of 180mm at f/4.5 and 348mm at f/4.9. (That’s the focal lengths, BTW.). I need something longer for Galaxy season. If I could afford faster versions of what I have, and if they were sharp across my sensor, they would be worth getting. For my regular terrestrial photography, I have about 9 different lenses. For both Astrophotography and Terrestrial photography, my first focus was on wider FOV, therefore physically small aperture diameter. So my scopes are only 40mm and 71mm. Per your concept, they are no good? With 26Mpixels printed at 13x19”, I have fantastic resolution for the big objects I am shooting. My two tiny scopes are reasonably “fast” for the tight spot diagrams they deliver over the APS image sensor I use.
@@swagonman Okay, let's look at it another way, using realistic telescopes upon which I just ran the math and a simulation to test the math. Imagine an 80 mm telescope at 400 mm FL giving f/5. Imagine a 400 mm telescope at 2000 mm FL, also giving f/5. Which do you think can put more energy on camera's sensor? The 80 mm telescope with its 5027 sq mm of space, or the 400 mm telescope with its 125,664 sq mm of space? Bear in mind that 2000 mm focal length telescope has a vastly narrower field of view. It makes for a bit of a conundrum, but if you want to see something like this in a practical test: just try shooting with a wide aperture lens vs a low aperture lens in low light and observe which image shows more detail and less noise. The results are going to show something pretty quickly and clearly: focal ratio is irrelevant, and aperture is king. So, while your comment that a lower focal length capturing light from a wider angle is technically correct, it has several flaws. Not least of which is the wider aperture always captures more light. But, of more relevance, in astrophotography we are generally going for highly directional light sources. And unless you are going for large light sources, like Milky Way backdrops or shooting one of the large complexes like the North America Nebula region, the wider FoV doesn't matter. Most of the things in space are very small light sources, almost point sources, and what matters is how much of their energy is put on the camera sensor, not how much of the total background is put on the sensor (unless your goal is also to shoot the background, but that is a subjective decision). Wide aperture puts more of the relevant light on the sensor regardless of goal, and wide aperture combined with appropriate focal length puts the light that matters in the spread over the pixels that matters most for detail. How focal ratio plays into this is irrelevant. It tells you little and contributes less. It's just a ratio. I like to think of it this way: Aperture is king; focal length is a choice. As to what you refer to as your "tiny refractors", lots of people have them and they make great images. I've been very happy with my own little refractor. But the day I put a wide aperture SCT on the mount and took my very first test image with it, I suddenly realized the truth of the adage: Aperture is king. But like I said in the video, focal ratio doesn't make one telescope better or worse than the other. Aperture sure does, and focal length depends on your photographic goal. But focal ratio is not a good way to compare telescopes at all, and that's the whole point of the video. You can still shoot good images with your little refractors because modern tech allows us to gather and stack light, so it compensates to a degree. That's just reality. If you take that as some slight against your refractors, that has more to do with you than me or this video.
I've been ruminating about this in my head for a month or so, thank you for finally explaining it clearly! I really enjoy these explainers you do, it scratches that curiosity! Thank you so much!
Thank you, though in retrospect I see I left a couple errors in the video that create confusion. I forgot to note that the camera lenses section was a thought experiment and the 25 mm lenses are not real. (I hope no one wastes time trying to buy them lol). I've been thinking in terms of telescopes so long that I had a brain infarction and forgot the number at the front of a camera lens relates its focal length instead of aperture. This area of focal ratio and the interplay of all the variables is one of those areas that is a perpetual head-scratcher because all the variables are interrelated in ways that are frequently complex and not intuitive. Change one little variable and so much else changes.
You're missing something critical here: light concentration. Equal focal ratios will have equal concentration of light on the image sensor. A 4 inch f/10 and a 14 inch f/10 scope will have equal exposure levels in equal length exposures with the same camera. The primary difference will be field of view, with the larger scope and it's longer focal length having the narrower field of view. But while the larger aperture IS gathering more light, it's also spreading that light over a larger area of the image sensor. The end result is that the images would be the same as far as apparent brightness of the object, you just see a narrower view with more detail with the larger scope. The larger scope with its longer focal length and smaller field of view also increases the effects of minor inconsistencies in your mount's alignment, tracking, and guiding. This means your exposure times are going to be limited to shorter exposures before things like periodic error become noticeable. A refractor with a shorter focal ratio, even if the aperture is relatively small , say 80 mm, is often a better option for imaging unless you need a narrower field of view. In which case you go with the larger aperture and corresponding longer focal length, but try to stick with a lower focal ratio as it increases the concentration of light on the sensor giving you the ability to gather more light in shorter exposure times.
You are describing focal ratio to a tee, and this, in point of fact, is what focal ratio indicates: The ratio of the concentration of light. Thus, a 50 mm aperture telescope with a focal length of 100 mm has a f/2 focal ratio, and a 200 mm aperture telescope with a 400 mm focal also has a f/2 focal ratio. Because focal ratio simply descrbies the light spread by the focal length/aperture relationship, it's only meaningful result is that brightness between the two telescopes remains consistent between these two wildly different telescopes. Nonetheless, brightness is not the same as energy and the two telescopes have wildly different capabilities despite a common and meaningless focal ratio. The 200 mm aperture telescope captures 16 times the light energy of the 50 mm aperture telescope; 16 times the photons. The photons are information, and this information means several things. One is the 200 mm aperture telescope will resolve much more detail. Two is the 200 mm telescope will present much less noise with an equivalent exposure time. Three is the 200 mm telescope will have a much tighter field of view so it will not capture the same light as the 50 mm aperture scope. Four: all those extra photons mean more options. For example, you could spread those photons four times wider at the image circle and get much more effective magnification complete with detail. That detail would be lost by the so-called "fast' telescope. Fast isn't really doing anything but putting photons in a smaller space. It gets you nothing extra and costs heavily in noise and optical demands. The faster the scope, the more the slightest imperfection or misalignment of optics matters. Bear in mind, the video states near the outset that it is more useful to define "fast" not as brightness but as photon energy captured. The video does not state that a slow scope produces a brighter image in a few pixels. The video reiterates toward the end that the term "fast" really should be dropped because it's so meaningless. Keep "focal ratio", sure. It has meaning. "Fast" is deception. The term creates the delusion that a "fast" telescope is a better scope. "Fast" comes at a heavy price--loss of focal length and resolution. The issue of easier guiding with a "fast" telescope is not an issue of focal ratio but of focal length. It is also an issue that is far, far less meaningful these days. Modern guiding and mounts are so much better than even just 5 years ago. Ultimately, "faster" is not better. In comparing telescopes, it has very little useful meaning at all.
@@SKYST0RY I agree that the larger aperture is collecting more photons. No question there. But since we're talking imaging, look at the amount of that energy that each pixel is receiving -- i.e. exposure. Because the light is being condensed into a smaller area, more photons are being concentrated on individual pixels than with the larger aperture. Equal focal ratios essentially mean equal exposure levels. Yes, with the larger aperture you're getting more total energy, but you're spreading it over a larger area, so the per-pixel exposure will differ. Here's a thought experiment: take two telescopes with equal apertures, say 100mm. One telescope has a focal length of 500 mm (f/5) the other 1,000 mm (f/10). Attach identical cameras to them and point them at the same target. Our hypothetical target is a face-on spiral galaxy that is essentially circular from our point of view. In the f/5 scope, it covers a portion of the image sensor that's exactly 100 pixels wide. Solving for area, we find that the target covers an area of about 7,854 pixels. Now, let's say that in a one-minute exposure each pixel detects an average of 10 photons per second for a total average exposure of 600 photons per pixel per minute and a grand total of 4,712,389 photons from the target object collected across the sensor. Now let's look at what happens in the f/10 scope. Since the focal length is doubled, the diameter of the object on the image sensor is doubled to 200 pixels. This gives us an area of 31,416 pixels. However, since aperture dictates the total amount of light we capture, the total number of photons received will be the same, 4,712,389. In a 60 second exposure this amounts to 150 photons per pixel on average with each pixel receiving an average of 2.5 photons per second. This means that the exposure level in the f/10 scope is 1/4 that of the f/5 scope. In order to attain the same level of exposure, the f/10 scope would need a 240 second (4 minute) exposure time. Yes, the scale of the image in the f/10 scope is larger, but it's fainter. Another way to look at this is to take a flashlight and stand in a dark room a couple of feet from a wall. Point the flashlight at the wall and observe the brightness and size of the spot. Now step back several feet and repeat the process. The size of the spot on the wall will have grown larger, but it will be fainter. The flashlight is still putting out the same amount of light, but it is now being spread over a larger area. When we change the aperture, we are changing the total amount of light collected. But if we retain the same focal ratio, we have the same concentration of that light. The size of the target object on the image sensor will change, and with it the average exposure for each pixel. If you have an 8 inch f/10 scope and a 4 inch f/10 scope, equal exposure times will yield equal exposure levels, just on a different image scale. However, if you use a shorter focal ratio, you will get more exposure. My f/5 72 mm refractor captures more light PER PIXEL faster than my 8 inch f/10 refractor. However, my 8 inch f/4 Newtonian captures more light per pixel than both of them.
@@henryv1598 Yup, by all means he forget that faster focal ratio telescope has a wider field of view, and thus collecting more photons over the sky, while bigger aperture scope collecting more photons per square area of the scope. I think his mistake is that he's comparing telescopes with the camera lens - an diaphragm will off course shorten the angle of the view of the faster lens, and thus collect less light, which is not the case with an telescope. It is as simply as that.
you mix up the brightness of the image, or how many photons land on a specific pixel, with the light-gathering ability of a telescope. And “fast” refers to a telescope that achieves the desired image brightness “fast". And f2.0 achieves that much faster as f4.0 oder f8.0.
I think you are referring to aperture vs etendue, though I am unsure. Etendue really needs to be covered for a complete picture, but this video would have turned into an hour or more if I had added that rabbit hole to it. It is important, just can only do so much per video. My suggestion would be to experiment. Using the same camera, try shooting some short exposure subs of a dim DSO, such as the Dumbbell Nebula, Wizard Nebula, Pacman Nebula, etc, with two telescopes of very different aperture but the same focal ratio then assess the outcomes for quality of information and noise content.
In this video, you did a great job demonstrating that it's not the focal ratio that matters, but rather the diameter at the light entry point. Many thanks!
Thank you. It is an imperfect video, though. I need to revisit the topic and illustrate this in a different way to clarify some things. The biggest hurdle seems to be the common misunderstanding that brighter equals more energy or that the wider field of view of a "fast" focal ratio telescope is providing more energy when in fact both telescopes are capturing the same field of view, hence same amount of energy, they just are portraying it differently.
@@astromeatric But his video is wrong on this. Focal ratio always matters and is correctly correlated to “fast” optics and “brightness”. He is confusing “information” with “resolution”. Bigger scopes with longer focal lengths and bigger apertures are needed to get high “resolution” on small objects. However, they do not deliver more “information”. Compared to a smaller scope with same focal ratio, they deliver the same amount of information. The bigger scope give higher resolution over a smaller FOV. The smaller scope gives less resolution over a larger FOV. Same total amount of photons per time, and same amount of information. Just like with any photography, you need different scopes/lenses for different subjects. There are great images to be made at every focal length (FOV). Scientists typically use very large scopes only because they need to make images and discoveries that hobbyists can’t afford. That’s how they can justify grant money. But even tiny scopes can make scientific discoveries. I just watched a video from a hobbyist, and it seems his image indicates that one object is closer than a background object. NASA wasn’t sure on which was closer, but had said it was likely the other way around. Anyway, this particular video is simply wrong and should be corrected. Lots of the comments agree. But the author seems quite arrogant about it. And he is enjoying lots of clicks due to the controversy, so it isn’t likely he will remove it. He should have added the word “terrifying” in the title for more click-bait. Yes, I’m being overly cynical. Sorry. And actually, the comment he left you indicates that I may be completely wrong about him, so I might have to apologize to him if/when he reposts.
Pixel size for image scale is just as important. A 2.9um pixel gathers 60% the light a 3.76um pixel does. And an F5.6 scope is half as bright to the sensor as a F4 scope. If you're using a popular camera based on 3.76um pixels, an 8inch F4 scope is a great choice. About 1" per pixel. Imaging at 0.5" per pixel at f8 means taking 4x the exposure length to get the same SNR. Guiding long enough to get enough SNR for faint detail is going to be much longer on a higher f ratio scope. Having to take 900 Sec subs is no fun. Speed - resolution - aperture.
Yes. This. If DSO object sampling space is the same, only aperture matters. If image sampling is the same (i.e., taking the same camera and putting it on a different telescope), then only f-ratio matters. Both need to be taken into account.
I am sure you are right. I often don't think about these things due to where I am. Due to the dark skies, the main source of noise I have to contend with in building SNR is just read noise which is so minimal these days. But as Cuiv once said, we are after SNR in astrophotography. I should cover this in a future video though I try to avoid issues that will be faced only by urban astrophotographers as I have literally no experience dealing with light pollution other than moonlight. However, the fact that cameras with larger pixels make a significant difference in exposure times is a universal. Leading to the trade off between pixel size and light capture. I noted in a previous video (can't remember which) that I didn't think being over sampled was so much of an issue anymore. I get great results with my somewhat over sampled Ares-M camera with 3.76 um pixels on the Celestron C8. I wonder if this would not work so well in a light polluted area.
@@mikehardy8247 B9. Ouch! I can hardly imagine. Though I did meet a girl from an Asian city once who told me she had never even seen the stars till she grew up and traveled.
It’s good you’re trying to correct some of the misconceptions in how people think of telescope attributes. One additional element is that although a larger aperture scope does capture more light if a target fits in it’s narrower field of view even if it’s f-ratio is larger, but if looking at the collection of photons from the overall sky, the lower f ratio scope will collect more total photons even if it captures fewer target photons simply due to its much larger field of view. This is the factor that confuses people and it should be addressed. The other factor is people also reference how fast a camera pixel fills with light which is also higher for the low f-ratio scope even though this isn’t a very important attribute anymore. So there are more elements that need to be covered here I’m afraid.
You're absolutely right. Some of these elements were addressed in a previous video: Understanding Focal Length: Trading Speed for Detail. There is so much to every little aspect of all these rabbit holes that going into them in depth would require a video of hours. Ultimately, with a "fast" scope, one has to decide between little image (sacrificing detail) for speed and/or lots of background vs slow scope (accepting photon spread but getting mostly the energy from the subject of interest on the sensor). More photon capture (as from a "fast" scope) is meaningless unless it's the photons we want. Even using a larger pixel camera to make better use of the photons has its price--some loss of resolution. It's all a trade off. No right choice, just choices.
Very interesting video. I did the math as well. I own an 8" RC which has a 44% obstruction of the primary mirror because of the secondary mirror. The open surface of the primary mirror is still about 50% bigger than on my 102mm Apo refractor. That's somewhat amazing and shows that nothing is as important on a telescope as aperture except having even more aperture. 😁 I think I also need a flattner reducer for my 8" to boost it a bit more. As you now with your 8" SCT sometimes it is difficult to get a nice framing using the native focal length under which I also suffer with my 8" RC. I have the RC to do some close-ups. I like those like you do. Recently I started to use to high focal lengths for targets intentionally to create the not that frequently seen pictures. I really like that. I hope I am going to have a good number of clear nights in winter as I would like to roam Orion with high focal length creating some stunning views.
Same here, I started with an Esprit 100 (great scope) but wanted something with a bit more focal length for smaller targets so bought a 12" RC (with 0.8x reducer which brings it down to F6.4). Even though the RC is 'slower' than the Esprit it absolutely destroys it in pure light gathering power. In addition, I can do binning on the RC which helps even more.
Long live aperture, the kink! LOL But your story reminds me of the day I took my 80 mm refractor (which is in its own right a great scope) off the mount and put on my first large reflector. The very first image I shot, at three times the focal length and a higher focal ratio, I was blown away. The SCT had captured so much detail of the subject so quickly. And it never stopped. My love of refractors died that day and I became all about reflectors with their wonderful wide aperture.
As a daytime photographer, I was surprised when I went to buy my first telescope that the "stats" didn't highlight the focal ratio. Aperture diameter was featured instead. Now I know why.
Aperture is infinitely more important than focal ratio. Really, aperture and focal length should be considered each on their own merits most of the time.
I have a C8 SCT and was “warned” as a newcomer to this hobby I should’ve purchased a small refractor but I’ve overcome the high focal length hurdles. I researched SCTs and started with an OAG for guiding with ASI174mm, an APS-C sized sensor camera, ASI071MC Pro. I purchased the Starizona focal reducer. The only wrong purchase was the AVX mount but I’m getting by with the mount. I’ve learned how to adjust the backlash and have improved the guiding. I’ve been imaging galaxies and I’m now shooting smaller nebula, e.g. PacMan. For wider nebula instead of purchasing a refractor I’ll probably purchase a Hyperstar, 390mm at F/1.9 in the future. Still using Siril and Sirilic for processing to keep it simple. I use NINA and an ASIAir Plus. I like most of my results but use Astrobin to see how others process the same htarget. I’m looking into duo narrow band filters. Like normal photography, somebody always has a better photo and I’m not in competition in my retired life. My friends and family like the photos I share. The biggest obstacle to improvement is lack of clear skies.
I had similar experiences, but soon found the hurdles with a high focal length were manageable and more than worth it. In addition to the clear obstacle, there is the frustration with clear skies during a full moon.
What will really blow your mind is if you ever get your hands on a hyperstar from Starizona. You can get F2 on an 8 inch Schmidt Cassegrain and I’ll tell you what that’s amazing. I have a 1972 vintage C8 that I use with a hyperstar and I can get some truly amazing images with it and then when I want to do high magnification I can just put the secondary mirror back in and image from the back. They are the most versatile Telescope ever made.
@@wesmagyar Wayne from SkyShed has shown me some of the images he took with his C14 RASA, which is a similar concept. The quality was crazy. I think if I ever went that route, I would just buy a dedicated telescope. It takes so long to get a setup working at its best, that once I get it all to my satisfaction I hate to change it.
@@SKYST0RY makes sense. I have some videos on my RUclips using it and I post some of my stuff to Astrobin. I’m an Over the road truck driver. So I can never get my setups to perfection. The vibrations in the truck make that near impossible. I’d love to get a local job where I could do a permanent setup like a pier or something at home. But living in Florida the jobs don’t pay enough…
Nothing at all wrong with a small refractor. If you're after a low focal length, you will get better detail resolution and noise performance from a good Newtonian because they just capture more light energy at the same focal length. But even the really good Newtonians can be somewhat tedious. A good refractor, in my experience, is fairly trouble-free.
I'm all about fast-as-possible, due to the rarity the clear skies have become over New England, but now I'm backtracking slowly, because once the hype about the F ratio died down, I want my round-star shapes back. And that's the other con of chasing the low F number. Image will suffer and dialing in the correct backfocus will be more difficult. Low F numbers also have the tendency to highlight any sort of optical issues and make them look worse than they are. Such as tilt or pinched optics or being slightly out of collimation. All these have to be considered, because there is always a trade off.
The Space Koala did a great video on exactly this. The lower the f ratio, the more any faults in the optical system will be amplifed. Persons I know who have RASAs frequently talk of frustrations with get them working right due to that.
I've thought about this a lot, too, and even experimented with a number of formulae to try to express telescope quality. Some I even derived myself. In the end, I came to see that aperture and focal length need to be considered separately to get a meaningful qualitative measure. Combined into focal ratio, they contribute little useful information about a telescope.
You can't beat aperture, but these 2 telescopes aren't really comparable. When people care about speed, they're usually comparing 2 telescopes of a similar aperture and design. Most amateur astrophotographers getting into the hobby are going to be limited by their tracking mount, and often have to travel to dark locations to escape light pollution. Someone in the market for a compact fast refractor isn't going to be comparing it to a bulky, long focal length Schmidt-Cassegrain.
We all are limited by our mounts, the foundation upon which success rests. If I had my druthers, I would steer persons new to AP away from refractors, too. They may get ease of use, but at the cost of resolution. But it depends on what they want, in the end.
Thank you! It's far from a perfect video but hopefully pushes the main idea: focal ratio is not a good way to judge between telescopes. What you want to do with it matters so much more, and in that light the best characteristics to consider for aperture and focal length, which I feel is usually best done separately.
You should definitely feel better. If you're targets are small DSOs, a long focal length with as much aperture as you can afford is best. While fans of "fast" focal ratios go on and on about the additional light coming in because of the wide field of view, what gets overlooked is whether that wider field of view is relevant. If they want the background, then it is worth it to them. If they want as much of the detail on the subject as possible, then the wider field of view and all its extra photons are wasted. And the price for that waste is a smaller subject with less detail. Focal ratio mattered little in either choice. The right focal length was key and the largest aperture one can afford makes it better.
I know that aperture is paramount in astrophotography for gathering light and I understand what f-ratio is, but in regard to the 50 and 24mm lenses, I'm not following. If I shoot with a 50mm at f4 and then change to a 24mm at f4, the proper exposure is exactly the same, so the same amount of light is indeed hitting he sensor. I can change to almost any focal length lens with the same results. I do this daily in my work. The old sunny 16 rule doesn't change with regard to focal length. Maybe I'm misunderstanding your example?
I don't know what camera you are using, but when I set my Fuji XT-3 to use its full sensor light meter mode and change from a 70 mm aperture lens to a 50 mm aperture lens, the camera will in fact change the shutter speed. I am providing a link to a test shot where I just did this. The test shot was shot at fixed settings: ISO 800, f/8 and SS 2. When I switched the camera to automatic shutter, it consistently wanted to increase SS by 0.2 to 0.3 seconds for the 50 mm lens, reflecting that the sensor is receiving less light. If you look at the two images that the link goes to, you will see the image shot with the 50 mm lens shows less detail and more noise, also consistent with less light. This is not really noticeable, however, unless shooting in low light conditions. yt3.ggpht.com/8tGTaOW7gMcpyrn4O2xI47xWHKgpwGLRqPIXt3WZWEg4jf5pev5c48eRl5KMn4ASl4FV9cYg_U3lMQ=s1600-rw-nd-v1 More to add now that I have a moment's reprieve the 100 other tasks: So, that small difference is probably more related to the optics or perhaps inaccuracies in the lens or light meter. Ultimately, the focal ratio is a ratio that expresses the spread of light within an optical device like a lens or telescope. If aperture changes, to maintain focal ratio, focal length must change. Ergo, the light in the image circle spreads more and this leads to a camera reporting the same exposure suggestions whether the lens is of narrower or wider aperture. The wider lens always captures more light, but if the focal ratio is the same, the light was spread more by a longer focal length at the image circle and the exposure requirements remain consistent. It creates the illusion that a f2 small aperture lens or telescope provides as much energy as a wider f2 lens. (Really, any two same focal ratios you choose). But the smaller f2 lens is just spreading less light more tightly, and the bigger f2 lens is spreading more light more widely, so the brightness looks consistent. It is important to understand that brightness is not the same as available energy. The difference in available energy shows up in noise and detail. The bigger the lens, the less noise and the more detail. Hence, a narrow aperture telescope's f2 is not the same as a wider aperture telescope's f2, and a wider aperture telescope will always capture more light no matter how fast a small aperture telescope is purported to be.
@@SKYST0RY I've been using 35mm, 120, 4x5 and 8x10 film cameras since the 80's and now digital Nikon, Canon and Sony since around 2005. From studios to location work, the f-stop is the same for any lens at the same shutter speed or flash setting. There may be tiny differences between lenses, (T-Stops will account for that), but less than 1/3 stop for good quality lenses. There are many variables when shooting normal photographs that affect exposure when changing lenses. The composition of the scene may change your settings if in auto mode, as you change focal lengths - but it has nothing to do with the transmission of the light to the sensor. It's only your camera being fooled by objects that may be brighter or darker that are now in the changed composition.. Using built-in metering can be fooled easily and is not the best way to judge proper exposure. Of course, this has nothing to do with astrophotography, just trying to clear up some confusion pertaining to f-stops and focal length.
You keep talking about gathering light. Yet you fail to mention that you have to record that light. The 50mm lens of the same f ratio spreads the light over 4 times the same area as the 25mm lens, so it is in fact no faster. F-ratio is exactly how you determine how fast an optical system is compared to another. This exactly why f-ratios are used and noted. The only other thing to take into account when determining how fast a system is are the losses from dissimilar optics such as reflectivity and light transmission.
I agree with @chrisfreerksen3050. F ratio is all about the speed of the telescope. As an example an 8" RASA will gather the same amount of light from M51 as an 8" SCT, but it will focus this light into a smaller image that is proportionally brighter and allows shorter exposures. This basic geometry applies across telescope types, but ignores transmission losses, as mentioned above. T-stops, as used in Cine lenses, is better and takes losses into account and is even better.
You are exactly right about the 50 mm focal length lens spreading light around more, though as it has twice the aperture, it has four times the light to spread. It's why the camera sensor reports the same shutter speed and exposure specs (more or less, though other factors such as optics efficiency will play a role in the real world). One might say the lower focal length lens can capture a wider field of light (which is true) but if it's not light from the subject, then it's useless light in many cases. The difference is not only nullified but the 50 mm lens provides more light energy allowing more possibilities because light is information. The 50 mm is also capable of capturing more detail and less noise in the low light where we astrophotographers usually work. Hence, as the saying goes, aperture is king, and an equivalent focal ratio does not make the two lenses of the illustration in any way equal. Ergo, focal ratio is a poor way to compare telescopes of different make and especially different aperture. To illustrate, I have attached a link to a rather poor looking image on my post page. In it, I am using a 50 mm vs a 70 mm camera lens, both at the same focal ratio, to shoot some stained wood in the shadows of my hallway. RUclips is lousy for relating low light images but the difference still shows. With all the other settings the same, the wider aperture produces a cleaner, more detailed image. ruclips.net/channel/UC5SkATJUS1n0unbR8IjwYnQcommunity?lb=UgkxY7ApQV_aveMgEOC_H8kwsZp2zmB0Gb-V
I’m sorry, but I don’t agree with the assertions made in this video. If you have a telescope with a 280mm aperture, and you image a nebula at 2,800mm focal length (f/10), you’ll need 25x the integration time to get the same exposure as shooting with a 280mm aperture at 560mm focal length (f/2)-assuming the same camera for both. I know this because I have an 11” SCT, which is natively f/10, and a HyperStar reducer, which brings the focal length to f/2, and the HyperStar makes an incredible difference! Furthermore, I have a buddy who has an 8” SCT with a HyperStar, and when we both shoot at f/2 with the same camera, our required integration times are the same-his targets are just smaller in the field of view. So, while it’s true, that my larger telescope collects more photons from the target than his, his concentrates the photons he does collect into fewer pixels in the camera and, therefore, achieves the same “brightness” with the same integration time. This is why f-ratio is, in fact, a crucial aspect of astrophotography. On an identical camera, a faster f-ratio will require less integration time.
Totally okay to disagree. I need to revisit this topic and try to explain my point more clearly. But you are correct. An f/10 telescope will require 25x exposure time than an f/2 telescope. The price for the supposed speed is the loss of focal length, hence magnification, hence relevant light and associated detail. That's a heavy price. In other words, if your target is something small, like the Dumbbell Nebula--which I recently made an extremely detailed image of with a high focal length/moderate focal ratio telescope which you can see on Astrobin, if you like--you get a tiny subject and a lot of background if you go for the "fast" focal ratio at great expense to your focal length. If you wanted that background, such an exposure is useful to you and the focal ratio was worth it. If you wanted the Dumbbell in as much detail as possible, the background is simply wasted light and most of the sensor is wasted space, ergo the focal ratio has wasted valuable focal length. Thus, the lower focal ratio isn't better, it just is. Regardless, since both the telescopes in your illustration are of the same aperture they are capturing the same amount of light sans regard of their focal ratio. How much light is captured is purely based on aperture and things like optical efficiency, not focal ratio. The "slower" focal ratio telescope just spreads the light around more. Regardless, the same amount of energy is there. You can use a camera with a sensor with larger pixels to take advantage of this spread (at the loss of detail) or accept the spread and use a camera with a sensor with smaller pixels and capture more detail of your subject matter at the cost of integration time. I know this is contrary to popular thinking that "fast" focal ratios put more energy on the sensor, but they don't. Fast focal ratios concentrate the energy captured by the aperture into a smaller image circle. This creates brightness in a few pixels at the expense of detail in many pixels; the energy captured remains the same. Regarding your second note: Your friend with an 8" with a Hyperstar vs your 280 mm telescope ( will call it 11" for easier comparison), yes, you will both experience the same integration times. Focal ratio is a ratio between aperture and focal length. It refers to how the Ap/FL relationship spreads light around. It doesn't define it, it just describes it. Increase aperture and you always capture more light, but if the focal length is increased to maintain the same focal ratio, then the light spread is consistent. Hence, your integration times will be the same (barring, of course, any differences in optics quality or sky transparency that may affect imaging). Regardless, your 280 mm is capturing a lot more light than his 203 mm aperture. In fact, your telescope has almost twice the surface area of mirror. Where this difference will become visible is in detail resolved, provided your optics, filters, sky conditions, etc, allow you to make use of that difference. Compare your images vs his for fine detail. I guess one might say the point of the video is faster doesn't mean more effective.
A wanderful explanation to the phenomenon I noticed ... I was imaging through my C11 and was able to livestack ( no guider setup then on the rig ) a really obscure galaxy pretty well .... at F10 I shoud have no chance ... meanwhile on my F7 115/800 using the same camera that particular object must have been about 120-150 pixels in total ... and in spite of having a guider and guiding at 0.5 with 180 seconds exposures ... I still preffered the Lucky imaging livestack to the "faster scope" ... So size beats speed ... hmmm sounds like boxing or MMA :)) 😁
You are describing the fact that aperture is king. I felt the same wonder the first time I took off my moderately "fast" 81 mm refractor and put an SCT on the mount and imaged my first target. The detail and intensity of the image were incredible.
Hi, your point is that f-ratio is not the diameter and the fact that diameter is directly linked to photons entering the scope. I am not sure in your approach how you deal the fact that longer focal length only consider rays comming only from a tiny area of the sky; what f-ratio does, it makes clear that both the size of area of the sky emitting photons actually collected by the scope AND the diameter of the scope are important to compute the absolute values of photons emitted by the source captures by the sensor. I agree on the fact that f ratio is overlooked but not to say ot has not the meaning commonly understood: * focal length is very well understood as its direct impact on framing is obvious; * Diameter is not enough looked at in particular for its impact it has on resolution (and price, weight and size...).
Diameter of aperture is such an important quality of a telescope that I feel it deserves a video entirely unto itself. Aperture is king; focal length is a choice. Focal ratio--that's the weird uncle that rarely has a job. I think, in retrospect, the best way to state my point is considering focal ratio tells you very little of use. It has some relevance in imaging but it's a poor way to choose between telescopes and the marketed idea that "fast" is better is nonsense. You are essentially correct--a lower focal length telescope will obtain more light from a wider field of view. What gets lost in the math, though, is the relevance. Math is an interpretation of reality, and all interpretations are imperfect and subject to context and use. In this case, one of the key things that gets lost is the wider field of view only benefits you if you want what's in that wider field of view. If your subject is a small DSO, all that wider field of view is not only wasted energy (even if it lands on the camera sensor), but it means your subject of interest will lack detail. Put simply--a wider field of view is not by default a benefit. If the wider field of view of a "faster" focal ratio is not your subject of interest, the wider field of view is only wasted potential.
Well this is more of a comparison between different types of scopes with different aperture sizes. When comparing them there are differences. Of course a wider aperture reflector gathers more light, but as stated in the video when comparing a telescope or lens of the same type it most assuredly is no myth. F ratio is no myth and would be pointless to note if this was the case. You will always collect data faster on a lower f ratio telescope when compared to a same type scope with higher f ratio and yes focal length does correspond with f ratio and is used appropriately for task at hand with proper conditions.
It's not so much that focal ratio is a myth. It definitely is a real thing (or, rather, a real relationship). They myth/s are more about the misconceptions around what focal ratio means.
Nice video. I think it's important to think about aperture. But you also have to consider pixel size. The Astro Imaging Channel had a great video on this called "The Quest for Aperture: Why Are Big Telescopes Better?" by Dr. John Hayes around the 45 minute mark. If the sampling rate is the same, then yes, the diameter of the telescope is the only thing that matters for "speed." However, if like most astrophotographers, you're only using one camera with the same pixel size on different telescopes, then it's only the f-ratio that matters. You have to consider how the focal ratio "spreads the butter out" as you've put it in previous videos. With a high f-ratio telescope with a larger aperture, the sampling per pixel is higher. That slows things down.
@@SKYST0RY My hopes and dreams are that someone smart out there will release the math to process multi segmented mirror data, so that us common plebs can build our own version of a tiny Magellan telescope and be able to actually turn something into an image.
@@MatthewHolevinski Wouldn't that be awesome. I have my hands full with the next observatory I am preparing to build. I don't know if I would ever dare tackling building a telescope. My field of science requires a fair background in probability and statistics. Juggling this optical physics stuff is a learning process for me. I love the field, but one can only do so much.
@@SKYST0RY Ya, I know a multi segmented mirror would be pure junk after building it, but gosh darnit I think it would be so cool, EVEN if the images were also junk. Would just be a fun thing to do.
IMHO is a very bad idea in these type of videos to use examples with f-ratios in cameras, since their lens are specified by focal length instead of aperture. The usual result at the end is a chaos. Probably showing how the image forms in the focal plane for a short-long f-ratio in a given scope aperture would clarify better the concept of magnification and the pixel-scale.
It's funny you would say this because I woke up this morning with virtually the exact same thought. Jung called these moments synchronicity. I had hoped to make the point really clear with this video but then woke up this morning thinking if I had created a diagram showing light spreading within telescopes of different aperture that would more clearly have explained my point. Seems like I will be covering this topic again in the future.
Your assertions are wrong and I am not a troll. Imagine that you have a testing lab with an evenly illuminated light panel wall and three 8 inch telescopes…a RASA at f/2, an 8” newt at f/4 and a 8” SCT at f/10. You attach the same camera to each and take a 2 second image. Which one would be brightest? Are they all the same because they are all 8” telescopes? No. The RASA image is brightest and the SCT is dimmest. F-ratio is real, it matters and it explains this result. Now bring in a camera lens, set it to f/2, attach your lab camera to the lens, and the brightness of the 2 second image will match the RASA image. Set the lens to f/4 or f/10 and it’ll match the newt and SCT brightness. Since you have telescopes, cameras and lenses you can test this out for us and make a video. You have to use the same camera in all tests because pixel size and sensor efficiency are also variables in the equations.
It's fine. Disagreeing is not trolling. But I think you are missing the fact that the focal ratio is a ratio, not a quantitative measure. Faster telescopes don't give you more light, they give you more concentrated light in smaller image circles. This allows for faster exposure, but it is not the same as more light. An f/10 8" SCT conveys exactly the same amount of light all the way to its image circle as an f/2 8" RASA if pointed at the same object under the same conditions. The RASA just produces a smaller image circle. If you were to use a larger camera sensor to capture all the light coming through the SCT, you would find just as much light energy in its image circle, just more spread out. Think of it this way. If light is a pat of butter, and you put it on a small piece of toast, you have more butter over the small area. That's a RASA. If you put the pat on a very large piece of toast, you have less butter over the larger area. That's the SCT. But it's still the same pat of butter. What this means in physical reality is that a f/2 telescope isn't actually brighter than a f/10 telescope. Rather, its image circle is simply more concentrated. This gives the benefit of faster exposure times, but it comes with heavy costs: loss of detail and much greater difficulty managing imperfections in the optical system. A higher f/ratio telescope provides greater detail at the expense of more imaging time, but this can be circumvented by use of a larger sensor. It is also far less likely to suffer as much from imperfections in the optical system. As this area proved confusing for some persons, I made a follow-up video: ruclips.net/video/5IKVoH0fTqc/видео.htmlsi=2WlcP91zjRTsvDpv
F ratio is meaningless in astrophotography. Field of view is obviously important (focal length). Aperture is important (resolving capability) (and diminishing bank balance!). Bigger is better but more expensive! An f4 250mm is better than a 50mm f4! otherwise we would all be using tiny scopes . Forget terrestrial photography jargon , lens diaphragms are for controlling depth of field which we are not concerned about .
Shh! This is the first part of my evil plan for world domination! Once I have them believing aperture is king, I am going to convince them Pluto is a planet!
at the end fast optics means fast optics , what are you talking about!!! in a fast optic you need less time to simply capture your subject. You specify it with the focal ratio number according to the optical focal length you need to use. again fast means fast!
I think this response is exactly why I made this video and will soon make another. F/2, for example, on 100 mm aperture OTA does not accomplish the same thing as F/2 on a 300 mm aperture OTA, because one fast isn't the same as another. Fast is a ratio, not a standard of measure.
Sorry, but the conclusion of this video is scientifically incorrect. You can easily check this with your flat panel and two telescopes with different f ratios. Look which one needs shorter exposures before saturation of your camera chip.
You present an interesting conundrum. Since telescopes do not have a changeable diaphragm and the decrease in brightness is only created due to how focal length narrows in on a smaller area of light emission in the sky, will laying a flat panel over any OTA effectively eliminate the validity of focal ratio altogether since in the case of both a low and high focal ratio telescope the entire optical surface would be covered with a light source? I'd love to see the results of the experiment. However, my suspicion is it will make a difference since I have run very similar tests using camera lenses which have the virtue of changeable entrance pupils over the same sensor. The differences don't really show up until illumination is very low; far lower than I would shoot flats at. But at low illumination--usually where we shoot astrophotography at--the camera wanted want to increase shutter speed to compensate for the lower diameter lens by an average of a few percent. I think at the darker levels the camera was just a bit more sensitive to different optics qualities--it wasn't a focal ratio issue. But the most important thing is the images shot in low light with the lower diameter lens will have decreased resolution and increased noise. I already did a demo shot of that, imaging stained wood in dark shadow with a 50 mm and 70 mm lens. You can see the outcome here: ruclips.net/channel/UC5SkATJUS1n0unbR8IjwYnQcommunity?lb=UgkxY7ApQV_aveMgEOC_H8kwsZp2zmB0Gb-V And Steven is right. Just take more exposures. In the end, focal ratio matters very little.
You made all kinds of assertions without providing any evidence to back up your claims. Let’s see same exposure length images through both the telescope systems you illustrated to see the differences you claim we should see. Forget what you think should happen based on some theoretical calculation. Let’s see what actually happens in real world results. As you say at the end, all kinds of other factors are at play. But if your “theory” was correct, then with your two lens examples, the 50mm and 25mm, each should require very different shutter speeds on the same scene even when set to the same f-ratio. That’s simply not true. It would mean that when I zoom a lens from 25mm to 50mm (such as with my 24-105mm f/4) but keep the f-stop constant, the exposure would change. That doesn’t happen. Why? Figure that out and you’ll realize why your “theory” is wrong. Again, actual examples, not just artwork and assertions, would have shown this. Test before theorizing on how you think things should work. Then present your evidence. Not just your arguments from authority. That’s science!
The experiments you are describing have already been conducted countless times ever since the dawn of photography. It is from these experiments that our mathematical rules that define the relationships between aperture and exposure are derived. But if you wish to repeat them, then do this: Shoot a target in low light using the same camera body and two lenses of different aperture. A great target is a park setting at sundown so you can get some varied brightness and shadow but all at low light. Make sure you use the same shutter speed, exposure, ISO, f stop and focal length. Then compare the detail and noise caught between the image shot with the narrow lens vs the detail and noise shot with the wide lens. Since I had a few minutes available earlier, I ran this as a demonstration for you using a Fuji XT-3 and a Fuji 18-135 lens and a Fuji 100-400 mm lens. The 18-135 mm lens has about a 50 mm aperture while the 100-400 mm lens has an aperture of about 70 mm. Camera settings at shutter speed 2, f/8, ISO 800. You can see the results in the attached link. Notice the image from the narrower aperture lens presents less contrast and more noise. This is due to less light being captured. You usually will not perceive this unless shooting low light, but in astrophotography we are working with extreme low light conditions in most cases. Why your camera shows you the same exposure and shutter speed settings is another matter I don't have time to delve into now. Perhaps in a future video. However, I can tell you that using my Fuji XT-3 with the camera's inbuilt light meter set to use the whole sensor, the camera most definitely wants to increase shutter speed when I switch from the 70 mm lens to the 50 mm lens. In the shooting conditions I shot this demonstration photo, it wanted to increase exposure time by +0.2 to +0.3. yt3.ggpht.com/8tGTaOW7gMcpyrn4O2xI47xWHKgpwGLRqPIXt3WZWEg4jf5pev5c48eRl5KMn4ASl4FV9cYg_U3lMQ=s1600-rw-nd-v1
@@SKYST0RY Thanks for the reply but your link takes me to two black frames. So I’m not sure what you’ve sent. Again, if you’re making assertions that go against what people might expect to be true then why not make the effort to include that evidence in your presentation, to be more convincing. Rather than just arguing from authority and suppositions that this has all been done before “countless times,” so you don’t have to. Bigger scopes and longer focal lengths have the benefit of greater detail and resolution but not in themselves brighter images photographically. That, too, has been shown countless times. In fact, it was shown long ago in the film days that telescopes with longer slower focal ratios would actually reach a greater limiting stellar magnitude than fast scopes. The opposite of what people might expect. But carry on. We can agree to disagree. As a fellow Canadian it would also be nice to know who and where you are.
@@alandyer910 I have found in doing astrophotography that my images turn out well mainly because I deviated a great deal from what persons expect to be true and broke out of a lot of doctrine about how it's supposed to be done. But like most people from the bush, I don't spend 60 seconds in a month worrying whether I have conformed to anyone's expectations or done things how they were supposed to be done. However, in this matter, my point is focal ratio lingo is misleading. A "fast" telescope may provide a brighter image, but it doesn't provide a better image. A small, "fast" telescope doesn't capture more light energy than an ostensibly slow telescope of wider aperture, it just may make a few pixels brighter because of the way it concentrates light and pulls in background (which in photographic composition may be irrelevant to the subject). As Cuiv recently pointed out, fast focal ratio gets you twice the SNR increase with a modern CMOS sensor though at the expense of lower focal length, which is tantamount to loss of resolution. In the end, aperture is king, focal length is a choice, and focal ratio is the weird uncle that never found a job. Now, I'm not sure what you think an argument from authority is. An argument from authority is when a person cites an authority and by that virtue claims the argument is right. When you find the authority that I cited in the video, let me know. As I have stated in many of my videos, while I have a heavy scientific background, I am not an astronomer nor an astrophysicist. It's not my field; just an area of profound interest. I will inevitably make mistakes. For me, this journey is as much one of discovery as sharing. But I will apply my scientific background to test truisms and see if they hold true, and when they don't, I'll see if I can do something about it. One of those things is addressing this nonsense that "fast" telescopes are better telescopes.. I will not be running 'experiments" on this, however, as I would just be repeating other's work, and I have neither the time nor inclination. You, however, are welcome to do so. Or you may choose to build on other good sources' work. The Space Koala has a really excellent video on the myth of fast being better than I thought was very well put together. If you only see dark images in the link, try to view in 4K and turn up your brightness. The images will still be dark. They were shot in a shadowed area under the conditions that reveal the reality of shooting with differing apertures that nonetheless report the same focal ratio. In astrophotography, we shoot in low light and it's in this lack of saturation that matters such as lack of information and noise matter most.
You are talking complete rubbish I'm afraid. What you are completely misunderstanding is that f# is as you say, focal length divided by aperture, but what you don't see is what the equation is telling you. f# is NORMALISED TO APERTURE so it actually doesn't matter what the aperture is for the purposes of defining "speed". I have worked with optical systems varying from f#2 to f#10 both SCT and refractors for over 20 years and have never seen any aperture dependence (independent of focal length) on speed. You can follow 20 years worth of collected knowledge on this subject on my website where I have several articles discussing what "speed" actually is.
Normally, I don't bother replying to trolls but you are so full of yourself, I can't help it in your special case. I have looked over your little home observatory. You aren't special. A lot of people have them. I have two and am building another one. But, unlike you, I didn't build from a kit. I designed and built my own, everything from the remote operating system to the hurricane resistant double walls. You have a higher education in sciences. Big whoop! So do I, though I first dipped my feet in university at age 11. I've been and worked with academics and scientists much of my life, from decent folk to those with egos so big it was about to make their heads pop (rather like you, I suspect), and trust me, the latter never impressed me. Narcissism doesn't impress anyone. And I'll post mark this by noting I cut my teeth in astrophotography when I was a kid and first strapped a camera onto the refractor I scrimped for and only paused my passion for AP during the many years I was in university, so, frankly, I really am unimpressed by your "20 years" of experience. Now, if we're finished the pissing match, I'll add that if you watch the video you'll understand the point of the video isn't that a bigger aperture makes for faster light gathering, it makes for more detail. I have looked over your technically proficient but otherwise unremarkable images (including your macros and micros) and concluded you may not fully understand this, but in other words, many things matter more than focal ratio, which has become by and large a marketing technique. Now, I have an itchy trigger finger when it comes to trolls, so the next round of snideness and you'll go poof from this channel forever. Adieu.
@@SKYST0RY Of course a larger aperture means more detail that just comes from 1.22 lambda/D, if you are saying that you didn't mean that aperture was behaving in the way I said you did - then that wasn't at all clear from your video. If I am the only one (quite possible) that completely misunderstood what you were saying - then I apologise unreservedly.
Thanks! Good analysis, and you probably saved me money chasing that fast scope I see ads for. 😁
I am glad it could help. So many things matter so much when considering purchasing a telescope.
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The 25mm f/4 collects the same amount of light as the 50mm f/4. Yes, the aperture diameter and area are smaller, but the 25mm is collecting light from a wider FOV. So it collects the photons from objects that are outside the FOV for the 50mm. When shooting an evenly illuminated field, both these focal lengths collect the same number of photons per unit of time. And even if we look at a finite sized object, such as Jupiter, the 50/4 will spread the photons over more pixels. The 25/4 will concentrate the photons onto 4x fewer pixels, although it is collecting 4x fewer photons from Jupiter due to smaller aperture size. Therefore, Jupiter’s brightness measured by the camera sensor will have the same brightness for both systems. The 50/4 simply enlarges objects for higher resolution while the 25/4 covers a wider FOV. Both are the same bright. Well, only if their T values are the same, but we are ignoring that in this example. In your example of two scopes, one has a large central obstruction that is not considered in the F value (aperture ratio). When we refer to how “bright” an optic is, we do need to consider this, as well as the transmissivity of the glass elements and reflectivity of any mirrors. Hence the T value.
You're quite right, of course. I should have specified in the video that in the thought experiment with the fictitious 25 mm lenses, they are 25 mm aperture, not focal length.
@@SKYST0RY Regardless, an f/4 lens is the same “fast” no matter the dimension of focal length or aperture. Focal ratio sets the “fastness”.
@@swagonman Let's try another thought experiment: Imagine a lens of 1 mm aperture and 2 mm focal length. It gives us a focal ratio of f/2. Imagine another lens of 100 mm aperture and 200 mm focal length, so it also has a focal ratio of f/2. Our 100 mm aperture lens has 7,854 times the area of the 1 mm aperture lens. While we might say that the tiny lens can make some pixels on a camera sensor bright as quickly as the big lens can make some pixels on a sensor bright, they are not equal. Not by a long shot. In point of fact, the much wider lens moves far more light energy and can affect far more pixels, quickly making a complete image while the 1 mm lens would struggle to illuminate a dot. Thus, we see that the 1 mm lens' f/2 rating is not the same as the 100 mm lens' f/2 rating. None of this even takes into account the greater ability of a wider aperture to resolve detail. Too much for this one rabbit hole.
In my posts section, at the suggestion of another person, I did an actual experiment (really a demonstration since this has long since been experimented on) and used a lens of 50 mm aperture and a lens of 70+ mm aperture with the same ISO, shutter speed and and f-stop setting to produce an image of the same object with the same camera body. At equivalent f-stop (focal ratio), the differences in the images are obvious. The bigger lens moves more photons, leading to more information and less noise. Focal ratio did not equally indicate what their performance would be. All things being equal, aperture is what mattered.
And that is the point of this video. Focal ratio is not a standard of measure; it is simply a ratio. It is useless as a tool for comparison, except in comparing a lens of a given aperture to itself or other lenses of similar aperture and other characteristics, and even then there are limits.
@@SKYST0RY This is silly. Of course a 2mm f/2 is a different lens than a 200mm f/2. They have vastly different FOV. But they are equally as “fast”. You really ought to take down your video, modify heavily, and repost it. As you wrote it, it is simply incorrect. You should focus on the difference of FOV versus Resolution. Just like terrestrial photography, different focal lengths are needed for different FOV. I have Astrographs of 180mm at f/4.5 and 348mm at f/4.9. (That’s the focal lengths, BTW.). I need something longer for Galaxy season. If I could afford faster versions of what I have, and if they were sharp across my sensor, they would be worth getting. For my regular terrestrial photography, I have about 9 different lenses. For both Astrophotography and Terrestrial photography, my first focus was on wider FOV, therefore physically small aperture diameter. So my scopes are only 40mm and 71mm. Per your concept, they are no good? With 26Mpixels printed at 13x19”, I have fantastic resolution for the big objects I am shooting. My two tiny scopes are reasonably “fast” for the tight spot diagrams they deliver over the APS image sensor I use.
@@swagonman Okay, let's look at it another way, using realistic telescopes upon which I just ran the math and a simulation to test the math. Imagine an 80 mm telescope at 400 mm FL giving f/5. Imagine a 400 mm telescope at 2000 mm FL, also giving f/5. Which do you think can put more energy on camera's sensor? The 80 mm telescope with its 5027 sq mm of space, or the 400 mm telescope with its 125,664 sq mm of space? Bear in mind that 2000 mm focal length telescope has a vastly narrower field of view. It makes for a bit of a conundrum, but if you want to see something like this in a practical test: just try shooting with a wide aperture lens vs a low aperture lens in low light and observe which image shows more detail and less noise. The results are going to show something pretty quickly and clearly: focal ratio is irrelevant, and aperture is king.
So, while your comment that a lower focal length capturing light from a wider angle is technically correct, it has several flaws. Not least of which is the wider aperture always captures more light. But, of more relevance, in astrophotography we are generally going for highly directional light sources. And unless you are going for large light sources, like Milky Way backdrops or shooting one of the large complexes like the North America Nebula region, the wider FoV doesn't matter. Most of the things in space are very small light sources, almost point sources, and what matters is how much of their energy is put on the camera sensor, not how much of the total background is put on the sensor (unless your goal is also to shoot the background, but that is a subjective decision). Wide aperture puts more of the relevant light on the sensor regardless of goal, and wide aperture combined with appropriate focal length puts the light that matters in the spread over the pixels that matters most for detail. How focal ratio plays into this is irrelevant. It tells you little and contributes less. It's just a ratio. I like to think of it this way: Aperture is king; focal length is a choice.
As to what you refer to as your "tiny refractors", lots of people have them and they make great images. I've been very happy with my own little refractor. But the day I put a wide aperture SCT on the mount and took my very first test image with it, I suddenly realized the truth of the adage: Aperture is king. But like I said in the video, focal ratio doesn't make one telescope better or worse than the other. Aperture sure does, and focal length depends on your photographic goal. But focal ratio is not a good way to compare telescopes at all, and that's the whole point of the video. You can still shoot good images with your little refractors because modern tech allows us to gather and stack light, so it compensates to a degree. That's just reality. If you take that as some slight against your refractors, that has more to do with you than me or this video.
I've been ruminating about this in my head for a month or so, thank you for finally explaining it clearly! I really enjoy these explainers you do, it scratches that curiosity! Thank you so much!
Thank you, though in retrospect I see I left a couple errors in the video that create confusion. I forgot to note that the camera lenses section was a thought experiment and the 25 mm lenses are not real. (I hope no one wastes time trying to buy them lol). I've been thinking in terms of telescopes so long that I had a brain infarction and forgot the number at the front of a camera lens relates its focal length instead of aperture. This area of focal ratio and the interplay of all the variables is one of those areas that is a perpetual head-scratcher because all the variables are interrelated in ways that are frequently complex and not intuitive. Change one little variable and so much else changes.
You're missing something critical here: light concentration. Equal focal ratios will have equal concentration of light on the image sensor. A 4 inch f/10 and a 14 inch f/10 scope will have equal exposure levels in equal length exposures with the same camera. The primary difference will be field of view, with the larger scope and it's longer focal length having the narrower field of view. But while the larger aperture IS gathering more light, it's also spreading that light over a larger area of the image sensor. The end result is that the images would be the same as far as apparent brightness of the object, you just see a narrower view with more detail with the larger scope.
The larger scope with its longer focal length and smaller field of view also increases the effects of minor inconsistencies in your mount's alignment, tracking, and guiding. This means your exposure times are going to be limited to shorter exposures before things like periodic error become noticeable.
A refractor with a shorter focal ratio, even if the aperture is relatively small , say 80 mm, is often a better option for imaging unless you need a narrower field of view. In which case you go with the larger aperture and corresponding longer focal length, but try to stick with a lower focal ratio as it increases the concentration of light on the sensor giving you the ability to gather more light in shorter exposure times.
You are describing focal ratio to a tee, and this, in point of fact, is what focal ratio indicates: The ratio of the concentration of light. Thus, a 50 mm aperture telescope with a focal length of 100 mm has a f/2 focal ratio, and a 200 mm aperture telescope with a 400 mm focal also has a f/2 focal ratio. Because focal ratio simply descrbies the light spread by the focal length/aperture relationship, it's only meaningful result is that brightness between the two telescopes remains consistent between these two wildly different telescopes. Nonetheless, brightness is not the same as energy and the two telescopes have wildly different capabilities despite a common and meaningless focal ratio. The 200 mm aperture telescope captures 16 times the light energy of the 50 mm aperture telescope; 16 times the photons. The photons are information, and this information means several things. One is the 200 mm aperture telescope will resolve much more detail. Two is the 200 mm telescope will present much less noise with an equivalent exposure time. Three is the 200 mm telescope will have a much tighter field of view so it will not capture the same light as the 50 mm aperture scope. Four: all those extra photons mean more options. For example, you could spread those photons four times wider at the image circle and get much more effective magnification complete with detail. That detail would be lost by the so-called "fast' telescope.
Fast isn't really doing anything but putting photons in a smaller space. It gets you nothing extra and costs heavily in noise and optical demands. The faster the scope, the more the slightest imperfection or misalignment of optics matters.
Bear in mind, the video states near the outset that it is more useful to define "fast" not as brightness but as photon energy captured. The video does not state that a slow scope produces a brighter image in a few pixels. The video reiterates toward the end that the term "fast" really should be dropped because it's so meaningless. Keep "focal ratio", sure. It has meaning. "Fast" is deception. The term creates the delusion that a "fast" telescope is a better scope. "Fast" comes at a heavy price--loss of focal length and resolution. The issue of easier guiding with a "fast" telescope is not an issue of focal ratio but of focal length. It is also an issue that is far, far less meaningful these days. Modern guiding and mounts are so much better than even just 5 years ago.
Ultimately, "faster" is not better. In comparing telescopes, it has very little useful meaning at all.
@@SKYST0RY I agree that the larger aperture is collecting more photons. No question there. But since we're talking imaging, look at the amount of that energy that each pixel is receiving -- i.e. exposure. Because the light is being condensed into a smaller area, more photons are being concentrated on individual pixels than with the larger aperture. Equal focal ratios essentially mean equal exposure levels. Yes, with the larger aperture you're getting more total energy, but you're spreading it over a larger area, so the per-pixel exposure will differ.
Here's a thought experiment: take two telescopes with equal apertures, say 100mm. One telescope has a focal length of 500 mm (f/5) the other 1,000 mm (f/10). Attach identical cameras to them and point them at the same target.
Our hypothetical target is a face-on spiral galaxy that is essentially circular from our point of view. In the f/5 scope, it covers a portion of the image sensor that's exactly 100 pixels wide. Solving for area, we find that the target covers an area of about 7,854 pixels. Now, let's say that in a one-minute exposure each pixel detects an average of 10 photons per second for a total average exposure of 600 photons per pixel per minute and a grand total of 4,712,389 photons from the target object collected across the sensor.
Now let's look at what happens in the f/10 scope. Since the focal length is doubled, the diameter of the object on the image sensor is doubled to 200 pixels. This gives us an area of 31,416 pixels. However, since aperture dictates the total amount of light we capture, the total number of photons received will be the same, 4,712,389. In a 60 second exposure this amounts to 150 photons per pixel on average with each pixel receiving an average of 2.5 photons per second.
This means that the exposure level in the f/10 scope is 1/4 that of the f/5 scope. In order to attain the same level of exposure, the f/10 scope would need a 240 second (4 minute) exposure time. Yes, the scale of the image in the f/10 scope is larger, but it's fainter.
Another way to look at this is to take a flashlight and stand in a dark room a couple of feet from a wall. Point the flashlight at the wall and observe the brightness and size of the spot. Now step back several feet and repeat the process. The size of the spot on the wall will have grown larger, but it will be fainter. The flashlight is still putting out the same amount of light, but it is now being spread over a larger area.
When we change the aperture, we are changing the total amount of light collected. But if we retain the same focal ratio, we have the same concentration of that light. The size of the target object on the image sensor will change, and with it the average exposure for each pixel.
If you have an 8 inch f/10 scope and a 4 inch f/10 scope, equal exposure times will yield equal exposure levels, just on a different image scale. However, if you use a shorter focal ratio, you will get more exposure. My f/5 72 mm refractor captures more light PER PIXEL faster than my 8 inch f/10 refractor. However, my 8 inch f/4 Newtonian captures more light per pixel than both of them.
@@henryv1598 Yup, by all means he forget that faster focal ratio telescope has a wider field of view, and thus collecting more photons over the sky, while bigger aperture scope collecting more photons per square area of the scope. I think his mistake is that he's comparing telescopes with the camera lens - an diaphragm will off course shorten the angle of the view of the faster lens, and thus collect less light, which is not the case with an telescope. It is as simply as that.
you mix up the brightness of the image, or how many photons land on a specific pixel, with the light-gathering ability of a telescope. And “fast” refers to a telescope that achieves the desired image brightness “fast". And f2.0 achieves that much faster as f4.0 oder f8.0.
I think you are referring to aperture vs etendue, though I am unsure. Etendue really needs to be covered for a complete picture, but this video would have turned into an hour or more if I had added that rabbit hole to it. It is important, just can only do so much per video. My suggestion would be to experiment. Using the same camera, try shooting some short exposure subs of a dim DSO, such as the Dumbbell Nebula, Wizard Nebula, Pacman Nebula, etc, with two telescopes of very different aperture but the same focal ratio then assess the outcomes for quality of information and noise content.
In this video, you did a great job demonstrating that it's not the focal ratio that matters, but rather the diameter at the light entry point. Many thanks!
Thank you. It is an imperfect video, though. I need to revisit the topic and illustrate this in a different way to clarify some things. The biggest hurdle seems to be the common misunderstanding that brighter equals more energy or that the wider field of view of a "fast" focal ratio telescope is providing more energy when in fact both telescopes are capturing the same field of view, hence same amount of energy, they just are portraying it differently.
@@astromeatric But his video is wrong on this. Focal ratio always matters and is correctly correlated to “fast” optics and “brightness”. He is confusing “information” with “resolution”. Bigger scopes with longer focal lengths and bigger apertures are needed to get high “resolution” on small objects. However, they do not deliver more “information”. Compared to a smaller scope with same focal ratio, they deliver the same amount of information. The bigger scope give higher resolution over a smaller FOV. The smaller scope gives less resolution over a larger FOV. Same total amount of photons per time, and same amount of information. Just like with any photography, you need different scopes/lenses for different subjects. There are great images to be made at every focal length (FOV). Scientists typically use very large scopes only because they need to make images and discoveries that hobbyists can’t afford. That’s how they can justify grant money. But even tiny scopes can make scientific discoveries. I just watched a video from a hobbyist, and it seems his image indicates that one object is closer than a background object. NASA wasn’t sure on which was closer, but had said it was likely the other way around. Anyway, this particular video is simply wrong and should be corrected. Lots of the comments agree. But the author seems quite arrogant about it. And he is enjoying lots of clicks due to the controversy, so it isn’t likely he will remove it. He should have added the word “terrifying” in the title for more click-bait. Yes, I’m being overly cynical. Sorry. And actually, the comment he left you indicates that I may be completely wrong about him, so I might have to apologize to him if/when he reposts.
Pixel size for image scale is just as important.
A 2.9um pixel gathers 60% the light a 3.76um pixel does. And an F5.6 scope is half as bright to the sensor as a F4 scope.
If you're using a popular camera based on 3.76um pixels, an 8inch F4 scope is a great choice. About 1" per pixel. Imaging at 0.5" per pixel at f8 means taking 4x the exposure length to get the same SNR.
Guiding long enough to get enough SNR for faint detail is going to be much longer on a higher f ratio scope. Having to take 900 Sec subs is no fun.
Speed - resolution - aperture.
Yes. This. If DSO object sampling space is the same, only aperture matters. If image sampling is the same (i.e., taking the same camera and putting it on a different telescope), then only f-ratio matters. Both need to be taken into account.
I am sure you are right. I often don't think about these things due to where I am. Due to the dark skies, the main source of noise I have to contend with in building SNR is just read noise which is so minimal these days. But as Cuiv once said, we are after SNR in astrophotography. I should cover this in a future video though I try to avoid issues that will be faced only by urban astrophotographers as I have literally no experience dealing with light pollution other than moonlight. However, the fact that cameras with larger pixels make a significant difference in exposure times is a universal. Leading to the trade off between pixel size and light capture. I noted in a previous video (can't remember which) that I didn't think being over sampled was so much of an issue anymore. I get great results with my somewhat over sampled Ares-M camera with 3.76 um pixels on the Celestron C8. I wonder if this would not work so well in a light polluted area.
Fascinating topic.
Wish I had your problem. Bortle 9 here.
More, shorter exposures seems best for me.
@@mikehardy8247 B9. Ouch! I can hardly imagine. Though I did meet a girl from an Asian city once who told me she had never even seen the stars till she grew up and traveled.
Star hoping isn't possible. I can barely see Polaris. My backyard is dark, but the sky isn't.
I think Cuiv had it worse.
It’s good you’re trying to correct some of the misconceptions in how people think of telescope attributes. One additional element is that although a larger aperture scope does capture more light if a target fits in it’s narrower field of view even if it’s f-ratio is larger, but if looking at the collection of photons from the overall sky, the lower f ratio scope will collect more total photons even if it captures fewer target photons simply due to its much larger field of view. This is the factor that confuses people and it should be addressed. The other factor is people also reference how fast a camera pixel fills with light which is also higher for the low f-ratio scope even though this isn’t a very important attribute anymore. So there are more elements that need to be covered here I’m afraid.
You're absolutely right. Some of these elements were addressed in a previous video: Understanding Focal Length: Trading Speed for Detail. There is so much to every little aspect of all these rabbit holes that going into them in depth would require a video of hours. Ultimately, with a "fast" scope, one has to decide between little image (sacrificing detail) for speed and/or lots of background vs slow scope (accepting photon spread but getting mostly the energy from the subject of interest on the sensor). More photon capture (as from a "fast" scope) is meaningless unless it's the photons we want. Even using a larger pixel camera to make better use of the photons has its price--some loss of resolution. It's all a trade off. No right choice, just choices.
Very interesting video. I did the math as well. I own an 8" RC which has a 44% obstruction of the primary mirror because of the secondary mirror. The open surface of the primary mirror is still about 50% bigger than on my 102mm Apo refractor. That's somewhat amazing and shows that nothing is as important on a telescope as aperture except having even more aperture. 😁
I think I also need a flattner reducer for my 8" to boost it a bit more. As you now with your 8" SCT sometimes it is difficult to get a nice framing using the native focal length under which I also suffer with my 8" RC.
I have the RC to do some close-ups. I like those like you do. Recently I started to use to high focal lengths for targets intentionally to create the not that frequently seen pictures. I really like that.
I hope I am going to have a good number of clear nights in winter as I would like to roam Orion with high focal length creating some stunning views.
Same here, I started with an Esprit 100 (great scope) but wanted something with a bit more focal length for smaller targets so bought a 12" RC (with 0.8x reducer which brings it down to F6.4). Even though the RC is 'slower' than the Esprit it absolutely destroys it in pure light gathering power. In addition, I can do binning on the RC which helps even more.
Long live aperture, the kink! LOL But your story reminds me of the day I took my 80 mm refractor (which is in its own right a great scope) off the mount and put on my first large reflector. The very first image I shot, at three times the focal length and a higher focal ratio, I was blown away. The SCT had captured so much detail of the subject so quickly. And it never stopped. My love of refractors died that day and I became all about reflectors with their wonderful wide aperture.
As a daytime photographer, I was surprised when I went to buy my first telescope that the "stats" didn't highlight the focal ratio. Aperture diameter was featured instead. Now I know why.
Aperture is infinitely more important than focal ratio. Really, aperture and focal length should be considered each on their own merits most of the time.
I have a C8 SCT and was “warned” as a newcomer to this hobby I should’ve purchased a small refractor but I’ve overcome the high focal length hurdles. I researched SCTs and started with an OAG for guiding with ASI174mm, an APS-C sized sensor camera, ASI071MC Pro. I purchased the Starizona focal reducer. The only wrong purchase was the AVX mount but I’m getting by with the mount. I’ve learned how to adjust the backlash and have improved the guiding. I’ve been imaging galaxies and I’m now shooting smaller nebula, e.g. PacMan. For wider nebula instead of purchasing a refractor I’ll probably purchase a Hyperstar, 390mm at F/1.9 in the future. Still using Siril and Sirilic for processing to keep it simple. I use NINA and an ASIAir Plus. I like most of my results but use Astrobin to see how others process the same htarget. I’m looking into duo narrow band filters. Like normal photography, somebody always has a better photo and I’m not in competition in my retired life. My friends and family like the photos I share.
The biggest obstacle to improvement is lack of clear skies.
I had similar experiences, but soon found the hurdles with a high focal length were manageable and more than worth it. In addition to the clear obstacle, there is the frustration with clear skies during a full moon.
What will really blow your mind is if you ever get your hands on a hyperstar from Starizona. You can get F2 on an 8 inch Schmidt Cassegrain and I’ll tell you what that’s amazing. I have a 1972 vintage C8 that I use with a hyperstar and I can get some truly amazing images with it and then when I want to do high magnification I can just put the secondary mirror back in and image from the back. They are the most versatile Telescope ever made.
@@wesmagyar Wayne from SkyShed has shown me some of the images he took with his C14 RASA, which is a similar concept. The quality was crazy. I think if I ever went that route, I would just buy a dedicated telescope. It takes so long to get a setup working at its best, that once I get it all to my satisfaction I hate to change it.
@@SKYST0RY makes sense. I have some videos on my RUclips using it and I post some of my stuff to Astrobin. I’m an Over the road truck driver. So I can never get my setups to perfection. The vibrations in the truck make that near impossible. I’d love to get a local job where I could do a permanent setup like a pier or something at home. But living in Florida the jobs don’t pay enough…
@@wesmagyar That's difficult. Most of the truck drivers I know are under a lot of stress. I remember when being a trucker used to be a great job.
What a fantastic explanation. Thank you. I am just working through that very thought process as I consider purchasing a small refractor.
Nothing at all wrong with a small refractor. If you're after a low focal length, you will get better detail resolution and noise performance from a good Newtonian because they just capture more light energy at the same focal length. But even the really good Newtonians can be somewhat tedious. A good refractor, in my experience, is fairly trouble-free.
I'm all about fast-as-possible, due to the rarity the clear skies have become over New England, but now I'm backtracking slowly, because once the hype about the F ratio died down, I want my round-star shapes back. And that's the other con of chasing the low F number. Image will suffer and dialing in the correct backfocus will be more difficult. Low F numbers also have the tendency to highlight any sort of optical issues and make them look worse than they are. Such as tilt or pinched optics or being slightly out of collimation. All these have to be considered, because there is always a trade off.
The Space Koala did a great video on exactly this. The lower the f ratio, the more any faults in the optical system will be amplifed. Persons I know who have RASAs frequently talk of frustrations with get them working right due to that.
Essentially the square area of the optics. The product of the area and F-ratio probably can give a better metric.
I've thought about this a lot, too, and even experimented with a number of formulae to try to express telescope quality. Some I even derived myself. In the end, I came to see that aperture and focal length need to be considered separately to get a meaningful qualitative measure. Combined into focal ratio, they contribute little useful information about a telescope.
Bigger scopes pull in light faster. Regardless of focal length. The comparison isn't about fastness, it's about aperture.
Yep! Aperture is king, focal length is a choice. Focal ratio is only how the butter is spread.
You can't beat aperture, but these 2 telescopes aren't really comparable. When people care about speed, they're usually comparing 2 telescopes of a similar aperture and design. Most amateur astrophotographers getting into the hobby are going to be limited by their tracking mount, and often have to travel to dark locations to escape light pollution. Someone in the market for a compact fast refractor isn't going to be comparing it to a bulky, long focal length Schmidt-Cassegrain.
We all are limited by our mounts, the foundation upon which success rests. If I had my druthers, I would steer persons new to AP away from refractors, too. They may get ease of use, but at the cost of resolution. But it depends on what they want, in the end.
Great explanation. It clarified a few nagging questions in my mind!
Thank you! It's far from a perfect video but hopefully pushes the main idea: focal ratio is not a good way to judge between telescopes. What you want to do with it matters so much more, and in that light the best characteristics to consider for aperture and focal length, which I feel is usually best done separately.
Thank you, I feel better about my 900fl F9 for DSOs!
You should definitely feel better. If you're targets are small DSOs, a long focal length with as much aperture as you can afford is best. While fans of "fast" focal ratios go on and on about the additional light coming in because of the wide field of view, what gets overlooked is whether that wider field of view is relevant. If they want the background, then it is worth it to them. If they want as much of the detail on the subject as possible, then the wider field of view and all its extra photons are wasted. And the price for that waste is a smaller subject with less detail. Focal ratio mattered little in either choice. The right focal length was key and the largest aperture one can afford makes it better.
I know that aperture is paramount in astrophotography for gathering light and I understand what f-ratio is, but in regard to the 50 and 24mm lenses, I'm not following. If I shoot with a 50mm at f4 and then change to a 24mm at f4, the proper exposure is exactly the same, so the same amount of light is indeed hitting he sensor. I can change to almost any focal length lens with the same results. I do this daily in my work. The old sunny 16 rule doesn't change with regard to focal length. Maybe I'm misunderstanding your example?
I don't know what camera you are using, but when I set my Fuji XT-3 to use its full sensor light meter mode and change from a 70 mm aperture lens to a 50 mm aperture lens, the camera will in fact change the shutter speed. I am providing a link to a test shot where I just did this. The test shot was shot at fixed settings: ISO 800, f/8 and SS 2. When I switched the camera to automatic shutter, it consistently wanted to increase SS by 0.2 to 0.3 seconds for the 50 mm lens, reflecting that the sensor is receiving less light. If you look at the two images that the link goes to, you will see the image shot with the 50 mm lens shows less detail and more noise, also consistent with less light. This is not really noticeable, however, unless shooting in low light conditions. yt3.ggpht.com/8tGTaOW7gMcpyrn4O2xI47xWHKgpwGLRqPIXt3WZWEg4jf5pev5c48eRl5KMn4ASl4FV9cYg_U3lMQ=s1600-rw-nd-v1
More to add now that I have a moment's reprieve the 100 other tasks: So, that small difference is probably more related to the optics or perhaps inaccuracies in the lens or light meter. Ultimately, the focal ratio is a ratio that expresses the spread of light within an optical device like a lens or telescope. If aperture changes, to maintain focal ratio, focal length must change. Ergo, the light in the image circle spreads more and this leads to a camera reporting the same exposure suggestions whether the lens is of narrower or wider aperture. The wider lens always captures more light, but if the focal ratio is the same, the light was spread more by a longer focal length at the image circle and the exposure requirements remain consistent. It creates the illusion that a f2 small aperture lens or telescope provides as much energy as a wider f2 lens. (Really, any two same focal ratios you choose). But the smaller f2 lens is just spreading less light more tightly, and the bigger f2 lens is spreading more light more widely, so the brightness looks consistent. It is important to understand that brightness is not the same as available energy. The difference in available energy shows up in noise and detail. The bigger the lens, the less noise and the more detail. Hence, a narrow aperture telescope's f2 is not the same as a wider aperture telescope's f2, and a wider aperture telescope will always capture more light no matter how fast a small aperture telescope is purported to be.
@@SKYST0RY I've been using 35mm, 120, 4x5 and 8x10 film cameras since the 80's and now digital Nikon, Canon and Sony since around 2005. From studios to location work, the f-stop is the same for any lens at the same shutter speed or flash setting. There may be tiny differences between lenses, (T-Stops will account for that), but less than 1/3 stop for good quality lenses. There are many variables when shooting normal photographs that affect exposure when changing lenses. The composition of the scene may change your settings if in auto mode, as you change focal lengths - but it has nothing to do with the transmission of the light to the sensor. It's only your camera being fooled by objects that may be brighter or darker that are now in the changed composition.. Using built-in metering can be fooled easily and is not the best way to judge proper exposure. Of course, this has nothing to do with astrophotography, just trying to clear up some confusion pertaining to f-stops and focal length.
You keep talking about gathering light. Yet you fail to mention that you have to record that light. The 50mm lens of the same f ratio spreads the light over 4 times the same area as the 25mm lens, so it is in fact no faster. F-ratio is exactly how you determine how fast an optical system is compared to another. This exactly why f-ratios are used and noted. The only other thing to take into account when determining how fast a system is are the losses from dissimilar optics such as reflectivity and light transmission.
I agree with @chrisfreerksen3050. F ratio is all about the speed of the telescope. As an example an 8" RASA will gather the same amount of light from M51 as an 8" SCT, but it will focus this light into a smaller image that is proportionally brighter and allows shorter exposures. This basic geometry applies across telescope types, but ignores transmission losses, as mentioned above. T-stops, as used in Cine lenses, is better and takes losses into account and is even better.
You are exactly right about the 50 mm focal length lens spreading light around more, though as it has twice the aperture, it has four times the light to spread. It's why the camera sensor reports the same shutter speed and exposure specs (more or less, though other factors such as optics efficiency will play a role in the real world). One might say the lower focal length lens can capture a wider field of light (which is true) but if it's not light from the subject, then it's useless light in many cases. The difference is not only nullified but the 50 mm lens provides more light energy allowing more possibilities because light is information. The 50 mm is also capable of capturing more detail and less noise in the low light where we astrophotographers usually work. Hence, as the saying goes, aperture is king, and an equivalent focal ratio does not make the two lenses of the illustration in any way equal. Ergo, focal ratio is a poor way to compare telescopes of different make and especially different aperture.
To illustrate, I have attached a link to a rather poor looking image on my post page. In it, I am using a 50 mm vs a 70 mm camera lens, both at the same focal ratio, to shoot some stained wood in the shadows of my hallway. RUclips is lousy for relating low light images but the difference still shows. With all the other settings the same, the wider aperture produces a cleaner, more detailed image. ruclips.net/channel/UC5SkATJUS1n0unbR8IjwYnQcommunity?lb=UgkxY7ApQV_aveMgEOC_H8kwsZp2zmB0Gb-V
Adorable dog, and a great video.
He's my little buddy.
I’m sorry, but I don’t agree with the assertions made in this video.
If you have a telescope with a 280mm aperture, and you image a nebula at 2,800mm focal length (f/10), you’ll need 25x the integration time to get the same exposure as shooting with a 280mm aperture at 560mm focal length (f/2)-assuming the same camera for both. I know this because I have an 11” SCT, which is natively f/10, and a HyperStar reducer, which brings the focal length to f/2, and the HyperStar makes an incredible difference!
Furthermore, I have a buddy who has an 8” SCT with a HyperStar, and when we both shoot at f/2 with the same camera, our required integration times are the same-his targets are just smaller in the field of view.
So, while it’s true, that my larger telescope collects more photons from the target than his, his concentrates the photons he does collect into fewer pixels in the camera and, therefore, achieves the same “brightness” with the same integration time.
This is why f-ratio is, in fact, a crucial aspect of astrophotography. On an identical camera, a faster f-ratio will require less integration time.
Totally okay to disagree. I need to revisit this topic and try to explain my point more clearly. But you are correct. An f/10 telescope will require 25x exposure time than an f/2 telescope. The price for the supposed speed is the loss of focal length, hence magnification, hence relevant light and associated detail. That's a heavy price. In other words, if your target is something small, like the Dumbbell Nebula--which I recently made an extremely detailed image of with a high focal length/moderate focal ratio telescope which you can see on Astrobin, if you like--you get a tiny subject and a lot of background if you go for the "fast" focal ratio at great expense to your focal length. If you wanted that background, such an exposure is useful to you and the focal ratio was worth it. If you wanted the Dumbbell in as much detail as possible, the background is simply wasted light and most of the sensor is wasted space, ergo the focal ratio has wasted valuable focal length. Thus, the lower focal ratio isn't better, it just is. Regardless, since both the telescopes in your illustration are of the same aperture they are capturing the same amount of light sans regard of their focal ratio. How much light is captured is purely based on aperture and things like optical efficiency, not focal ratio. The "slower" focal ratio telescope just spreads the light around more. Regardless, the same amount of energy is there. You can use a camera with a sensor with larger pixels to take advantage of this spread (at the loss of detail) or accept the spread and use a camera with a sensor with smaller pixels and capture more detail of your subject matter at the cost of integration time. I know this is contrary to popular thinking that "fast" focal ratios put more energy on the sensor, but they don't. Fast focal ratios concentrate the energy captured by the aperture into a smaller image circle. This creates brightness in a few pixels at the expense of detail in many pixels; the energy captured remains the same.
Regarding your second note: Your friend with an 8" with a Hyperstar vs your 280 mm telescope ( will call it 11" for easier comparison), yes, you will both experience the same integration times. Focal ratio is a ratio between aperture and focal length. It refers to how the Ap/FL relationship spreads light around. It doesn't define it, it just describes it. Increase aperture and you always capture more light, but if the focal length is increased to maintain the same focal ratio, then the light spread is consistent. Hence, your integration times will be the same (barring, of course, any differences in optics quality or sky transparency that may affect imaging). Regardless, your 280 mm is capturing a lot more light than his 203 mm aperture. In fact, your telescope has almost twice the surface area of mirror. Where this difference will become visible is in detail resolved, provided your optics, filters, sky conditions, etc, allow you to make use of that difference. Compare your images vs his for fine detail.
I guess one might say the point of the video is faster doesn't mean more effective.
A wanderful explanation to the phenomenon I noticed ... I was imaging through my C11 and was able to livestack ( no guider setup then on the rig ) a really obscure galaxy pretty well .... at F10 I shoud have no chance ... meanwhile on my F7 115/800 using the same camera that particular object must have been about 120-150 pixels in total ... and in spite of having a guider and guiding at 0.5 with 180 seconds exposures ... I still preffered the Lucky imaging livestack to the "faster scope" ... So size beats speed ... hmmm sounds like boxing or MMA :))
😁
You are describing the fact that aperture is king. I felt the same wonder the first time I took off my moderately "fast" 81 mm refractor and put an SCT on the mount and imaged my first target. The detail and intensity of the image were incredible.
Hi, your point is that f-ratio is not the diameter and the fact that diameter is directly linked to photons entering the scope.
I am not sure in your approach how you deal the fact that longer focal length only consider rays comming only from a tiny area of the sky;
what f-ratio does, it makes clear that both the size of area of the sky emitting photons actually collected by the scope AND the diameter of the scope are important to compute the absolute values of photons emitted by the source captures by the sensor.
I agree on the fact that f ratio is overlooked but not to say ot has not the meaning commonly understood:
* focal length is very well understood as its direct impact on framing is obvious;
* Diameter is not enough looked at in particular for its impact it has on resolution (and price, weight and size...).
Diameter of aperture is such an important quality of a telescope that I feel it deserves a video entirely unto itself. Aperture is king; focal length is a choice. Focal ratio--that's the weird uncle that rarely has a job. I think, in retrospect, the best way to state my point is considering focal ratio tells you very little of use. It has some relevance in imaging but it's a poor way to choose between telescopes and the marketed idea that "fast" is better is nonsense.
You are essentially correct--a lower focal length telescope will obtain more light from a wider field of view. What gets lost in the math, though, is the relevance. Math is an interpretation of reality, and all interpretations are imperfect and subject to context and use. In this case, one of the key things that gets lost is the wider field of view only benefits you if you want what's in that wider field of view. If your subject is a small DSO, all that wider field of view is not only wasted energy (even if it lands on the camera sensor), but it means your subject of interest will lack detail. Put simply--a wider field of view is not by default a benefit. If the wider field of view of a "faster" focal ratio is not your subject of interest, the wider field of view is only wasted potential.
Or as we say...'Aperture is King!"
Absolutely! Aperture is kink; focal length is a choice.
Well this is more of a comparison between different types of scopes with different aperture sizes. When comparing them there are differences. Of course a wider aperture reflector gathers more light, but as stated in the video when comparing a telescope or lens of the same type it most assuredly is no myth. F ratio is no myth and would be pointless to note if this was the case. You will always collect data faster on a lower f ratio telescope when compared to a same type scope with higher f ratio and yes focal length does correspond with f ratio and is used appropriately for task at hand with proper conditions.
It's not so much that focal ratio is a myth. It definitely is a real thing (or, rather, a real relationship). They myth/s are more about the misconceptions around what focal ratio means.
There's no replacement for displacement.
Nice video. I think it's important to think about aperture. But you also have to consider pixel size. The Astro Imaging Channel had a great video on this called "The Quest for Aperture: Why Are Big Telescopes Better?" by Dr. John Hayes around the 45 minute mark. If the sampling rate is the same, then yes, the diameter of the telescope is the only thing that matters for "speed." However, if like most astrophotographers, you're only using one camera with the same pixel size on different telescopes, then it's only the f-ratio that matters. You have to consider how the focal ratio "spreads the butter out" as you've put it in previous videos. With a high f-ratio telescope with a larger aperture, the sampling per pixel is higher. That slows things down.
Absolutely right. I try to address these things one tidbit at a time, otherwise there is no end to how deep rabbit holes can go.
Aperture will always be king :)
No doubt. Aperture is king. Focal length is a choice. Focal ratio is the weird uncle who never got a job.
@@SKYST0RY My hopes and dreams are that someone smart out there will release the math to process multi segmented mirror data, so that us common plebs can build our own version of a tiny Magellan telescope and be able to actually turn something into an image.
@@MatthewHolevinski Wouldn't that be awesome. I have my hands full with the next observatory I am preparing to build. I don't know if I would ever dare tackling building a telescope. My field of science requires a fair background in probability and statistics. Juggling this optical physics stuff is a learning process for me. I love the field, but one can only do so much.
@@SKYST0RY Ya, I know a multi segmented mirror would be pure junk after building it, but gosh darnit I think it would be so cool, EVEN if the images were also junk. Would just be a fun thing to do.
IMHO is a very bad idea in these type of videos to use examples with f-ratios in cameras, since their lens are specified by focal length instead of aperture. The usual result at the end is a chaos. Probably showing how the image forms in the focal plane for a short-long f-ratio in a given scope aperture would clarify better the concept of magnification and the pixel-scale.
It's funny you would say this because I woke up this morning with virtually the exact same thought. Jung called these moments synchronicity. I had hoped to make the point really clear with this video but then woke up this morning thinking if I had created a diagram showing light spreading within telescopes of different aperture that would more clearly have explained my point. Seems like I will be covering this topic again in the future.
Your assertions are wrong and I am not a troll. Imagine that you have a testing lab with an evenly illuminated light panel wall and three 8 inch telescopes…a RASA at f/2, an 8” newt at f/4 and a 8” SCT at f/10. You attach the same camera to each and take a 2 second image. Which one would be brightest? Are they all the same because they are all 8” telescopes? No. The RASA image is brightest and the SCT is dimmest. F-ratio is real, it matters and it explains this result. Now bring in a camera lens, set it to f/2, attach your lab camera to the lens, and the brightness of the 2 second image will match the RASA image. Set the lens to f/4 or f/10 and it’ll match the newt and SCT brightness. Since you have telescopes, cameras and lenses you can test this out for us and make a video. You have to use the same camera in all tests because pixel size and sensor efficiency are also variables in the equations.
It's fine. Disagreeing is not trolling. But I think you are missing the fact that the focal ratio is a ratio, not a quantitative measure. Faster telescopes don't give you more light, they give you more concentrated light in smaller image circles. This allows for faster exposure, but it is not the same as more light. An f/10 8" SCT conveys exactly the same amount of light all the way to its image circle as an f/2 8" RASA if pointed at the same object under the same conditions. The RASA just produces a smaller image circle. If you were to use a larger camera sensor to capture all the light coming through the SCT, you would find just as much light energy in its image circle, just more spread out. Think of it this way. If light is a pat of butter, and you put it on a small piece of toast, you have more butter over the small area. That's a RASA. If you put the pat on a very large piece of toast, you have less butter over the larger area. That's the SCT. But it's still the same pat of butter.
What this means in physical reality is that a f/2 telescope isn't actually brighter than a f/10 telescope. Rather, its image circle is simply more concentrated. This gives the benefit of faster exposure times, but it comes with heavy costs: loss of detail and much greater difficulty managing imperfections in the optical system. A higher f/ratio telescope provides greater detail at the expense of more imaging time, but this can be circumvented by use of a larger sensor. It is also far less likely to suffer as much from imperfections in the optical system.
As this area proved confusing for some persons, I made a follow-up video: ruclips.net/video/5IKVoH0fTqc/видео.htmlsi=2WlcP91zjRTsvDpv
F ratio is meaningless in astrophotography. Field of view is obviously important (focal length). Aperture is important (resolving capability) (and diminishing bank balance!). Bigger is better but more expensive!
An f4 250mm is better than a 50mm f4! otherwise we would all be using tiny scopes . Forget terrestrial photography jargon , lens diaphragms are for controlling depth of field which we are not concerned about .
Absolutely agree! I wish I had thought to put it something like the way you just did: "At any focal ratio, the bigger aperture always wins."
In short, a big bucket will always gather more water than a smaller one in a given amount of time.
Succinctly correct.
F ratios cant be compared between different telescopes, let alone different kinds of telescope.
It's virtually pointless using it as a point of comparison, for sure.
At others have said, the logic in this video is completely wrong.
are you trying to stir up controversy for view count? because a lot of things said in this video are wrong
Shh! This is the first part of my evil plan for world domination! Once I have them believing aperture is king, I am going to convince them Pluto is a planet!
at the end fast optics means fast optics , what are you talking about!!! in a fast optic you need less time to simply capture your subject. You specify it with the focal ratio number according to the optical focal length you need to use. again fast means fast!
I think this response is exactly why I made this video and will soon make another. F/2, for example, on 100 mm aperture OTA does not accomplish the same thing as F/2 on a 300 mm aperture OTA, because one fast isn't the same as another. Fast is a ratio, not a standard of measure.
We had a telescope when I was in elementary school. It wasn't very fast. In fact it was rather stationary. (Booooo!)
This is just wrong, what you say… please do proper research…
Sorry, but the conclusion of this video is scientifically incorrect. You can easily check this with your flat panel and two telescopes with different f ratios. Look which one needs shorter exposures before saturation of your camera chip.
Just take more exposures ! And don't worry about a ratio it's unimportant.
You present an interesting conundrum. Since telescopes do not have a changeable diaphragm and the decrease in brightness is only created due to how focal length narrows in on a smaller area of light emission in the sky, will laying a flat panel over any OTA effectively eliminate the validity of focal ratio altogether since in the case of both a low and high focal ratio telescope the entire optical surface would be covered with a light source? I'd love to see the results of the experiment. However, my suspicion is it will make a difference since I have run very similar tests using camera lenses which have the virtue of changeable entrance pupils over the same sensor. The differences don't really show up until illumination is very low; far lower than I would shoot flats at. But at low illumination--usually where we shoot astrophotography at--the camera wanted want to increase shutter speed to compensate for the lower diameter lens by an average of a few percent. I think at the darker levels the camera was just a bit more sensitive to different optics qualities--it wasn't a focal ratio issue. But the most important thing is the images shot in low light with the lower diameter lens will have decreased resolution and increased noise. I already did a demo shot of that, imaging stained wood in dark shadow with a 50 mm and 70 mm lens. You can see the outcome here: ruclips.net/channel/UC5SkATJUS1n0unbR8IjwYnQcommunity?lb=UgkxY7ApQV_aveMgEOC_H8kwsZp2zmB0Gb-V
And Steven is right. Just take more exposures. In the end, focal ratio matters very little.
You made all kinds of assertions without providing any evidence to back up your claims. Let’s see same exposure length images through both the telescope systems you illustrated to see the differences you claim we should see. Forget what you think should happen based on some theoretical calculation. Let’s see what actually happens in real world results. As you say at the end, all kinds of other factors are at play.
But if your “theory” was correct, then with your two lens examples, the 50mm and 25mm, each should require very different shutter speeds on the same scene even when set to the same f-ratio. That’s simply not true. It would mean that when I zoom a lens from 25mm to 50mm (such as with my 24-105mm f/4) but keep the f-stop constant, the exposure would change. That doesn’t happen. Why? Figure that out and you’ll realize why your “theory” is wrong.
Again, actual examples, not just artwork and assertions, would have shown this. Test before theorizing on how you think things should work. Then present your evidence. Not just your arguments from authority. That’s science!
The experiments you are describing have already been conducted countless times ever since the dawn of photography. It is from these experiments that our mathematical rules that define the relationships between aperture and exposure are derived. But if you wish to repeat them, then do this: Shoot a target in low light using the same camera body and two lenses of different aperture. A great target is a park setting at sundown so you can get some varied brightness and shadow but all at low light. Make sure you use the same shutter speed, exposure, ISO, f stop and focal length. Then compare the detail and noise caught between the image shot with the narrow lens vs the detail and noise shot with the wide lens. Since I had a few minutes available earlier, I ran this as a demonstration for you using a Fuji XT-3 and a Fuji 18-135 lens and a Fuji 100-400 mm lens. The 18-135 mm lens has about a 50 mm aperture while the 100-400 mm lens has an aperture of about 70 mm. Camera settings at shutter speed 2, f/8, ISO 800. You can see the results in the attached link. Notice the image from the narrower aperture lens presents less contrast and more noise. This is due to less light being captured. You usually will not perceive this unless shooting low light, but in astrophotography we are working with extreme low light conditions in most cases. Why your camera shows you the same exposure and shutter speed settings is another matter I don't have time to delve into now. Perhaps in a future video. However, I can tell you that using my Fuji XT-3 with the camera's inbuilt light meter set to use the whole sensor, the camera most definitely wants to increase shutter speed when I switch from the 70 mm lens to the 50 mm lens. In the shooting conditions I shot this demonstration photo, it wanted to increase exposure time by +0.2 to +0.3. yt3.ggpht.com/8tGTaOW7gMcpyrn4O2xI47xWHKgpwGLRqPIXt3WZWEg4jf5pev5c48eRl5KMn4ASl4FV9cYg_U3lMQ=s1600-rw-nd-v1
@@SKYST0RY Thanks for the reply but your link takes me to two black frames. So I’m not sure what you’ve sent. Again, if you’re making assertions that go against what people might expect to be true then why not make the effort to include that evidence in your presentation, to be more convincing. Rather than just arguing from authority and suppositions that this has all been done before “countless times,” so you don’t have to. Bigger scopes and longer focal lengths have the benefit of greater detail and resolution but not in themselves brighter images photographically. That, too, has been shown countless times. In fact, it was shown long ago in the film days that telescopes with longer slower focal ratios would actually reach a greater limiting stellar magnitude than fast scopes. The opposite of what people might expect.
But carry on. We can agree to disagree. As a fellow Canadian it would also be nice to know who and where you are.
@@alandyer910 I have found in doing astrophotography that my images turn out well mainly because I deviated a great deal from what persons expect to be true and broke out of a lot of doctrine about how it's supposed to be done. But like most people from the bush, I don't spend 60 seconds in a month worrying whether I have conformed to anyone's expectations or done things how they were supposed to be done. However, in this matter, my point is focal ratio lingo is misleading. A "fast" telescope may provide a brighter image, but it doesn't provide a better image. A small, "fast" telescope doesn't capture more light energy than an ostensibly slow telescope of wider aperture, it just may make a few pixels brighter because of the way it concentrates light and pulls in background (which in photographic composition may be irrelevant to the subject). As Cuiv recently pointed out, fast focal ratio gets you twice the SNR increase with a modern CMOS sensor though at the expense of lower focal length, which is tantamount to loss of resolution. In the end, aperture is king, focal length is a choice, and focal ratio is the weird uncle that never found a job.
Now, I'm not sure what you think an argument from authority is. An argument from authority is when a person cites an authority and by that virtue claims the argument is right. When you find the authority that I cited in the video, let me know. As I have stated in many of my videos, while I have a heavy scientific background, I am not an astronomer nor an astrophysicist. It's not my field; just an area of profound interest. I will inevitably make mistakes. For me, this journey is as much one of discovery as sharing. But I will apply my scientific background to test truisms and see if they hold true, and when they don't, I'll see if I can do something about it. One of those things is addressing this nonsense that "fast" telescopes are better telescopes.. I will not be running 'experiments" on this, however, as I would just be repeating other's work, and I have neither the time nor inclination. You, however, are welcome to do so. Or you may choose to build on other good sources' work. The Space Koala has a really excellent video on the myth of fast being better than I thought was very well put together.
If you only see dark images in the link, try to view in 4K and turn up your brightness. The images will still be dark. They were shot in a shadowed area under the conditions that reveal the reality of shooting with differing apertures that nonetheless report the same focal ratio. In astrophotography, we shoot in low light and it's in this lack of saturation that matters such as lack of information and noise matter most.
You are talking complete rubbish I'm afraid. What you are completely misunderstanding is that f# is as you say, focal length divided by aperture, but what you don't see is what the equation is telling you. f# is NORMALISED TO APERTURE so it actually doesn't matter what the aperture is for the purposes of defining "speed". I have worked with optical systems varying from f#2 to f#10 both SCT and refractors for over 20 years and have never seen any aperture dependence (independent of focal length) on speed. You can follow 20 years worth of collected knowledge on this subject on my website where I have several articles discussing what "speed" actually is.
Normally, I don't bother replying to trolls but you are so full of yourself, I can't help it in your special case. I have looked over your little home observatory. You aren't special. A lot of people have them. I have two and am building another one. But, unlike you, I didn't build from a kit. I designed and built my own, everything from the remote operating system to the hurricane resistant double walls. You have a higher education in sciences. Big whoop! So do I, though I first dipped my feet in university at age 11. I've been and worked with academics and scientists much of my life, from decent folk to those with egos so big it was about to make their heads pop (rather like you, I suspect), and trust me, the latter never impressed me. Narcissism doesn't impress anyone. And I'll post mark this by noting I cut my teeth in astrophotography when I was a kid and first strapped a camera onto the refractor I scrimped for and only paused my passion for AP during the many years I was in university, so, frankly, I really am unimpressed by your "20 years" of experience. Now, if we're finished the pissing match, I'll add that if you watch the video you'll understand the point of the video isn't that a bigger aperture makes for faster light gathering, it makes for more detail. I have looked over your technically proficient but otherwise unremarkable images (including your macros and micros) and concluded you may not fully understand this, but in other words, many things matter more than focal ratio, which has become by and large a marketing technique. Now, I have an itchy trigger finger when it comes to trolls, so the next round of snideness and you'll go poof from this channel forever. Adieu.
@@SKYST0RY Of course a larger aperture means more detail that just comes from 1.22 lambda/D, if you are saying that you didn't mean that aperture was behaving in the way I said you did - then that wasn't at all clear from your video. If I am the only one (quite possible) that completely misunderstood what you were saying - then I apologise unreservedly.
What we have to decide what is the field of view .
Im gay and im telling you, bigger+more fat is better.