How to make a Field Map of ANY lens!

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  • Опубликовано: 2 фев 2025

Комментарии • 125

  • @eightmilesupwind9030
    @eightmilesupwind9030 2 года назад +10

    For the entire lecture, from the very beginning to the very end, I was staring at that F0.95 in your hand. It's so satisfying to learn about your final test result for that lens.

  • @johnwesleybrewer
    @johnwesleybrewer 2 года назад +11

    I always appreciate when I walk away from one of Ted’s videos I have zero need for gear acquisition. My skills always need more refinement and practice not dropping money on nonsense.

  • @alexanderkarnazes9195
    @alexanderkarnazes9195 2 года назад +2

    Hi, Ted. Thank you for walking viewers through one of your go-to lens tests. Using a tripod, how far off the ground do you usually set your camera for this type of test?

  • @AristeidisSfakianos
    @AristeidisSfakianos 2 года назад +1

    That's quite eye opening. Very often I find myself wondering whether my lenses are as sharp as when I got them. I shoot in rain and high humidity often and I am worried that this might have affected the sharpness if my lens. I'll give it a try!

  • @thomaseriksson6256
    @thomaseriksson6256 6 месяцев назад

    Thank you I just updated lenses to my new camera D850 so I will use this methods to check them

  • @frost8077
    @frost8077 2 года назад

    Subscribed! These videos that go deeper into the engineering and physics of lenses fascinate me.

  • @ShutterSounds
    @ShutterSounds 2 года назад +1

    Thanks Ted! I have a lot of vintage lenses and I will be out on the street today shooting test images to use your technique.

  • @chipfreundnc
    @chipfreundnc 2 года назад

    Ted, thank you for this video. I have been wanting to get in and analyze my lens, and haven't quite known the best / easiest approach. Your technique here makes perfect sense without having to get super technical.

  • @willbaren
    @willbaren 2 года назад

    I’d seen you do this before, I was going to look for it because I couldn’t remember the details, and then you repeated it with more details, for which I am grateful. Thank you so much :)

  • @andrewleece7567
    @andrewleece7567 2 года назад +1

    So much to learn about photography 😂
    could the find edges be used when doing lens calibration / micro adjustment ?

  • @SergioMusel
    @SergioMusel 2 года назад

    Does that mean that you do need a perfectly flat surface with texture and perfectly leveled camera? Roads tend to be wavy, so the precise distance to sensor of each point is different, resulting in an uneven "sharpness line". I understand that you need extremely controlled studio scene where you need to be sure that if you have some variances throughout the frame it is because of the lens, not because of it being out of focus due to different distance of the different parts of the subject. When having a curved line, it can just be the curvy road exaggerated by shallow depth of field

  • @virginiainla8085
    @virginiainla8085 2 года назад

    My gosh, you've sent me down a huge rabbit hole since I watched this! I've been testing all my lenses, old and new, just for fun. I am now waiting on film from my lenses that I only use on film cameras. Thanks! I think... 🤪😅🙃

  • @lehrerlampel9975
    @lehrerlampel9975 2 месяца назад

    Great (and useful) approach - thanks for sharing!

  • @llchan
    @llchan 2 года назад +1

    I have some questions regarding the process. Does it matter if I take the photo in RAW or JPEG? If it’s RAW, do I need to do any processing in, say, Lightroom, before I open it in Photoshop and apply the filter? For RAW, Lightroom automatically applies some sharpening. Should I disable that before taking it to PS?
    Great video. Keep up with the good work. I know this is an equipment video but I watched this channel because it’s not just about equipment.

  • @walontuoja4336
    @walontuoja4336 2 года назад

    As an avid retro lens shooter this is very useful as a metric to help me understand what to expect. Thanks!

  • @andreabeneforti4981
    @andreabeneforti4981 2 года назад

    Very interesting video! tomorrow I will test all of my lenses!

  • @supong_al_creation
    @supong_al_creation 2 года назад +1

    This is so interesting. Thank you for educating us

  • @chbrownie18
    @chbrownie18 2 года назад

    I have used a simpler method which focuses on the sensor resolution combined with the lens. I have two older 16MP bodies, a Canon 1Ds mark II and a Fuji X-Pro1. I use the kit 18-55mm zoom with the Fuji and with the Canon, the red 24-105mm zoom along with two Sigma Art lenses. If I take the shot, blow it up in Capture One and look at sharp vertical or horizontal lines in the subject images, it becomes really apparent in all of my cases that the lenses are as sharp as they need to be for these sensors. Single pixels line up along these edges on one side or the other and the result is binary. I had an older Canon 100mm Macro which didn't cut it but all of these lenses do. When I upgrade both of these bodies, I will then have a different set of criteria, but for now, any sharper lenses would be a waste.

  • @Nijensikkens
    @Nijensikkens 2 года назад +5

    Hi Ted I'm pretty interested how megapixel count impacts lens quality. For example I noticed that my Nikon 50mm 1.4G lens reders better on my 16MP Nikon Df then on my 45MP Nikon D850. Can you make an explanation how that works? Again great video 😊

    • @Mike_M_Smith
      @Mike_M_Smith 2 года назад

      I don’t have that problem with a similar setup.

    • @Nijensikkens
      @Nijensikkens 2 года назад +1

      @@Mike_M_Smith hmm strange maybe I just have a bad copy of the lens 🙄

    • @estoylisto
      @estoylisto 2 года назад +1

      Try to resize the 45MP picture to 16MP and compare the 2 .. using higher MP sensor adds more challenges to the lens so even small issues might get magnified and become obvious... the 50 1.4G isn't a very Sharp lens specially wide open so 45MP sensor will expose its shortcomings

    • @Nijensikkens
      @Nijensikkens 2 года назад

      @@estoylisto thanks for the tip I will try that 😊

  • @stephenblackwood3954
    @stephenblackwood3954 2 года назад

    Thanks! this is a great, quick way to analyze and compare the performance of multiple lenses of same focal length at same aperture. The part about using Find Edges in Photoshop is most helpful.

  • @rickfarber4243
    @rickfarber4243 2 года назад +1

    Thanks very much for this. I don't have Photoshop, but would the Focus Mask in Capture One work as well?

  • @jonfletcher147
    @jonfletcher147 2 года назад

    Always the voice of reason Ted….. superb.

  • @RJ-nk9qz
    @RJ-nk9qz 2 года назад

    Excellent info as usual. Just popping in to say thanks for this and ALL your videos. Your work is highly appreciated.

  • @Bob4golf1
    @Bob4golf1 2 года назад

    Excellent! I tried it with a bunch of my lenses and my Hasselblad (XCD series) proved to be better than most everything else - surprise. Great video - Thanks!

  • @drmatthewhorkey
    @drmatthewhorkey 2 года назад

    I learn sooooooo much watching all your videos

  • @kentvun
    @kentvun Год назад

    This a really good idea to test out sharpness and focal plane of any lens, thank you.
    I fingered simpler and albeit may be less accurate way of doing this, that is by using focus peaking of my em5 ii, I can visualise the focal plane of my lens when my aperture wide open and I try focus on a flat textured surface, i.e. my carpet floor, and I am able to visualise my focal plane. =)

  • @Nybykiosken
    @Nybykiosken 2 года назад

    The first thought when I saw the results for the Canon lens was ’Field Camera’. Great video Ted!

  • @hobbytake2845
    @hobbytake2845 2 года назад +1

    Excellent video, you’ve taken a very complicated subject and simplified it to a very understandable level. And given us the simple way to do it on our own. Thank you! Love the film simulations, still trying to finesse them to my liking but I’ll get there.

  • @jeffselles645
    @jeffselles645 2 года назад

    Hello, I did this exactly as stated in the video with Photoshop 2022 on a fractured street comparable or even better than the sample in the video. Over and Over testing, all I ever got was a very nice edge image but no focus above and below line - none, just beautiful, high contrast patterns of the street. What gives ? Is another step needed with the new Photoshop release ?

  • @RÅNÇIÐ
    @RÅNÇIР2 года назад +43

    Recently cut my finger on a shattered lens. So, very sharp.

  • @Sincerelytk
    @Sincerelytk 2 года назад +2

    This is absolutely amazing. I buy different lenses, typically used/ 2nd market, and know nothing about the lens. Or sometimes, I buy a used camera and the owner throws in lens or 2 and I have to do research on the lens to find it's sharpest point. Although this is more time consuming, this might be my best bet at understanding my gear. I almost resorted to using that black and white focal chart haha Thank you again!

  • @armstrongskyview2810
    @armstrongskyview2810 2 года назад +1

    This was a really good video, bit boring to begin but once you finally showed what was going on, not only did it make sense but it got me excited to get testing all my lens's tomorrow lol
    Thanks, a new sub :)

  • @CraigBergonzoni
    @CraigBergonzoni 2 года назад

    Awesome video, Ted. I've never tried this before but will definitely try it now.

  • @asamcqueen3513
    @asamcqueen3513 2 года назад

    Going to have to play with this with a lens like the Thambar, which is usually regarded for its impressionist painter look rather than sharpness. I find in practice though there's noticeable detail and contrast hanging out in the haze, which has me curious.

  • @vincencohan3626
    @vincencohan3626 2 года назад

    Can you please do this to all your fuji lenses? Thank you

  • @chrisambler9917
    @chrisambler9917 2 года назад

    Ted. Great vid as ever. Could you tell me how I do this lens check on Capture One pls? Thanks Chris

  • @jorgegildealmeida1985
    @jorgegildealmeida1985 2 года назад

    Hello Ted! I saw your video and tried your method to test one of my lenses but it was without a tripod so it's not conclusive, at least not for me, so I'll have to repeat it. You recommend that the test photo should be taken with the diaphragm all open and I think it's very good that way, so we can see the behaviour of the lens in its supposed worst aperture. But can't we take several photos at different apertures to see how the optical behaviour of the lens evolves as we close the diaphragm? Is there something I am missing? Best regards from Portugal. Jorge

  • @ddsdss256
    @ddsdss256 2 года назад +1

    Thanks, but are there alternative methodologies for those of us who refuse to rent Adobe's products (as there are better programs out there that you can actually own)? Also, wouldn't you want to test at different apertures to find "sweet" (and not so sweet) spots or would one expect to find no variation?

    • @fotograf736
      @fotograf736 2 года назад

      I use GIMP, does most of what PS does, albeit more painfully. There's a find edges function in its filters.

    • @robph8421
      @robph8421 2 года назад

      I test apertures with the focus peaking feature that all digital cams offer. Using any textured cardboard surface, you can see which aperture setting lights up the spikes best. The results are rather disappointedly uninteresting though: the “best” apertures are always the middling ones, f4, f5.6, f8.

    • @ddsdss256
      @ddsdss256 2 года назад +1

      @@fotograf736 Thanks! I may try that. I do have PaintShop Pro 2022 Ultimate which may have a similar function (I see that it does have an "Edge Seeker") plus I primarily use the DxO suite (PhotoLab/Film Pack/Nik), but I don't think they have such a function. First, I need to find the time to make some suitable test images!

  • @stuartflipflops
    @stuartflipflops 2 года назад

    Have you tried a road test on an Aero Ektar 178mm f2.5?

  • @cmichaelhaugh8517
    @cmichaelhaugh8517 2 года назад

    I’ve seen you do this before. Going to have to try it. Thank you.

  • @maze400
    @maze400 2 года назад

    Will this technique work for lens calibration?

  • @PS-lh1ml
    @PS-lh1ml 2 года назад

    Hey Ted, do you do any editing before applying the filter? Would that make any difference in the sense of ruining the test? I ask this because I accidentally got some deep shadow on a few of my test shots that are breaking the "lines" resulting from the "find edges".

  • @ThePhotofit
    @ThePhotofit 2 года назад

    Hi Ted. I learnt a lot from your video (as I do with most of yours). Here’s a question that might be interesting to debate and that is: Have we reached a point where lenses and camera sensors are “too sharp”. Could this be why the original Canon 5D is so highly regarded and could it also be the secret ingredient to achieving a ‘film look’ and why film is experiencing a renaissance?
    I’ve even experimented with own pixel shift technique whereby I duplicate a layer on an image and shift the canvas by a fraction to achieve a tiny amount of blur. The results are interesting!

  • @njihia
    @njihia 2 года назад

    Correct me if I am wrong, a key feature of cinema lenses is minimising fall off across the image that ends up in the sensor?

  • @PacoM.
    @PacoM. 2 года назад +1

    I love the presets Ted. Tried them out on Canon and Sony and I noticed straight out the of the camera they look better on Canon. I have an older Sony so the colors aren't as nice. But yes, the presets/profiles are great! Thank you so much.

    • @Levoiar
      @Levoiar 2 года назад +1

      Fancy seeing you here hahahaha

    • @PacoM.
      @PacoM. 2 года назад +1

      @@Levoiar 😅 its crazy how the internet makes it a smaller world.

  • @Zealor365
    @Zealor365 2 года назад

    Did I miss something? I don't recall hearing about resolving power for the lens. How high can that lens you are using resolve? Is "High" defined as the separation of two distinct points in the focal path of light reflection from the subject?

  • @bruce-le-smith
    @bruce-le-smith 2 года назад

    Great video, thank you!! I was just thinking, 'gee I have no idea how to test my lenses on my camera, I bet it's hard...' and here you've made it so easy! The bokah on that dream lens is so familiar from 60s album art, love it!

  • @soholingo
    @soholingo 2 года назад

    What lens are you holding when the video starts?

  • @greggorter
    @greggorter 2 года назад

    I thoroughly enjoy your videos Ted ! excellent, thank you for sharing your valuable knowledge. A question for you is “what is your opinion on testing with consumer lens calibration software”

  • @jimpurcell
    @jimpurcell 2 года назад

    Thanks for sharing! Very helpful!

  • @TimberGeek
    @TimberGeek 2 года назад +33

    This will be fun to play with but I'm perpetually at the "it is what it is" stage with my (99% used) gear.

  • @BilBrown
    @BilBrown 2 года назад +1

    The lens Eggleston was using and shot when I met him in 2018.

  • @MrCameraJunkie
    @MrCameraJunkie 2 года назад

    Amazing Breakdown, Great job 👏🏽😉

  • @tonyhayes9827
    @tonyhayes9827 2 года назад

    what an extraordinarily insightful video.

  • @JeffWernerIthacaNY
    @JeffWernerIthacaNY 2 года назад

    awesome and intuitive way to easily visualize a lens focal plane! I'm a bit worried that my favorite LTM and S-mount lenses might not perform favorably!

  • @christopherherrick703
    @christopherherrick703 2 года назад

    Is it possible to do this same test in Lightroom?

  • @AlGreenLightThroughGlass
    @AlGreenLightThroughGlass 2 года назад

    Brilliant - will happily borrow concept but credit you - calling it the 'Forbes' field mapping technique or test - for future old lens reviews

  • @pimpingkek4947
    @pimpingkek4947 2 года назад

    I've got the f1.9/80 mm xcd for hasselblad. Pretty sharp, allegedly

  • @enigmabletchley6936
    @enigmabletchley6936 2 года назад

    Very interesting, thanks for creating this video.

  • @Murdog36
    @Murdog36 2 года назад

    Great as always Ted!

  • @marcusoutdoors4999
    @marcusoutdoors4999 2 года назад

    No questions, but super useful testing method

  • @mr.t.9019
    @mr.t.9019 2 года назад

    Wow this was very helpful. Many thanks.

  • @patriot9455
    @patriot9455 2 года назад

    I have a Kiev 88 with a couple of lenses, including a 300mm lens. I also have a Minolta Maxxum. I use Ilford and Kodak, amongh others as they become available. is Ilford close to Kodak in the way it handles light. I shoot color, grayscale and true B&W. Being low on income, I have a run what ya brung concept. My film is going to be developed out of house, and will be "played with" in the latest version of Photoshop, as well as Lightroom and a few others in the Adobe system. At 71, I do not see me becoming an international sensation, but I could do some regional work and have a blast making a small amount here and there.

  • @robph8421
    @robph8421 2 года назад +1

    The 8mm movie lenses are actually very interesting for photos, and you can see why with this “pavement” method here. They were made for the micro movies way back when, so on conventional cams they will show a parabolic focus field. I hope more people will explore and talk about parabolic focused photos.

    • @sclogse1
      @sclogse1 2 года назад

      Are you saying that because the area of the surface of the 8mm frame is so small that the relative distance to the rear lens in one of those little Switars is closer to the same from all points on the film frame? Or that the directrix of the parabola is closer to the same length on 8mm than it is on a 35mm frame?

    • @robph8421
      @robph8421 2 года назад

      @@sclogse1 From the viewpoint of the photographer, the directrix line (or plane, if you consider the focus field a 3-D paraboloid) is farther beyond the nose. The rest of the parabola/loid extends backward. Those micro lens designers only concerned themselves with a tiny area at the nose. But conventional sensors will catch the wide parts of the thing, and it looks very interesting. 8mm movie lenses are good for this, but some standard lenses can also do it with the nearest element flipped backward. I posted photos of this kind on Facebook but they got bare scant attentions. Don’t know if I can find them again. I hope people will try this out and see.

  • @davidjones8070
    @davidjones8070 2 года назад

    Thanks for reminding me that I need to do this to some of my Tamron lenses to see how they fair in the test.

  • @stuartcarden1371
    @stuartcarden1371 2 года назад

    What a great video! Thank you so much. Very informative.

  • @qnetx
    @qnetx 2 года назад

    Very cool way to test. However, I’m not so much interested in the sharpness as much as the visual character and rendering. That is more subjective and personal. Your test does bring up an interesting point though. I shoot many wildlife images of birds and often compose them into one of the rule of thirds sweet spots. I also like to crop the frame to a 16:9 aspect ratio. I think I will run this test at the settings and distances that I typically shoot at and see how much I’m sacrificing. Thank you very much for sharing.

  • @j16m02
    @j16m02 2 года назад

    Really enjoyable and educational. Thanks

  • @JimEmbury
    @JimEmbury 9 месяцев назад

    Excellent! Thank you.

  • @jean-claudemuller3199
    @jean-claudemuller3199 2 года назад

    Well done Ted 👍

  • @guvenayaz
    @guvenayaz 2 года назад +3

    Is there a 'find edges' filter in capture one ?

    • @dougd2723
      @dougd2723 2 года назад

      Is it Focus Mask in C1?

  • @gregsimmons1709
    @gregsimmons1709 2 года назад

    That was *very* informative!

  • @MeAMuse
    @MeAMuse 2 года назад +8

    Shouldn’t this have been on Lensday :)
    Great job Ted… although I will say… I much prefer lenses with character. These days sharpness is overrated. I like lenses that forces / allows you to do creative things.

  • @FranHoganPanama
    @FranHoganPanama 2 года назад

    Very interesting. thank you!

  • @johnlane2511
    @johnlane2511 2 года назад

    OK Ted, this one is very helpful.

  • @rodbotic
    @rodbotic 2 года назад +4

    shouldn't we expect a "C" shape Depth of field since that is what the distance to the lens.
    Also the distortions would be because it is projecting onto a flat plane.(sensor).

    • @robph8421
      @robph8421 2 года назад +1

      Designers went through hoops to “flatten” out the focus. Some projection lens will show a nice curved field, and that’s because they were designed to catch the celluloid films that were alway curled.

  • @thebowlerusa1
    @thebowlerusa1 2 года назад

    Thank you, very educational!

  • @NobbyChariot
    @NobbyChariot 2 года назад

    Great video Ted👍

  • @Chris-NZ
    @Chris-NZ 2 года назад

    Incredibly interesting Ted and real world :)

  • @artistwithcameras
    @artistwithcameras Год назад

    Good stuff

  • @sundarAKintelart
    @sundarAKintelart 2 года назад

    MTF chart displayed/explained by way of a test shot of a paved road.

    • @sundarAKintelart
      @sundarAKintelart 2 года назад

      The plane of focus of the paved road could be taken diagonal to the film/sensor plane too.
      Also a back lit road, or a bright contrsty thin line say a rope (say a thin black rope against a pale background) would be better.

  • @SoCalTrojan
    @SoCalTrojan 2 года назад +1

    i used a file and sharpened the outer edges of my lenses and they have never been sharper!

  • @bcleste
    @bcleste 2 года назад +1

    Isn’t that the lens Zak Snyder used on army of the dead?

  • @andyjones5192
    @andyjones5192 2 года назад +2

    Interesting vid however how relevant is sharpness anyway? I mean its not rocket science to build a reasonable sharp lens in the 21st century but does it really matter? The dream lens is magnificent exactly for that reason. Not being prefect.

    • @Reason-fg4ik
      @Reason-fg4ik 2 года назад +2

      Ansel Adams wrote and talked about the concept of, "acceptably sharp. " That's good enough for me.

  • @sclogse1
    @sclogse1 2 года назад

    Everything changes when you start shooting in a square format. Corner lack of sharpness, edge falloff, vignetting, distortion, all head to Miami for the winter. By the way, I just watched a curator show all the lenses Kubrick used and had developed for his films. Another world. Then I typed in deep field (think Barry Lyndon) and the first thing that came up was a B&H page with a Fuji lens costing $19,000.

  • @EduardoVelezIII
    @EduardoVelezIII 2 года назад

    I think Pixelmator Pro users can use the "comics" effect (which is really just supposed to be a fun goofy filter), since it seems to use edge detection. With this effect, I seem to be able to tell that one of my lenses has a mustache-shaped focus plane when it's wide open and another lens has a slight curve. I suspect this is good enough for me to get an idea of how my lenses perform, but someone more knowledgeable about this kind of software might be able to tell us why this might not be as accurate as the "find edges" filter in PS.

  • @AjaySingh-228
    @AjaySingh-228 2 года назад

    Good Sir🙂✌️

  • @root9065
    @root9065 2 года назад +2

    I would agree with you and also argue that the more you know about a lens's performance, the more you can pull out of it. That said, sharpness is not everything, in fact, the sharpest lenses out there, paired with the highest res camera (Sony a1 for example), produce images that are too sterile, too clinical. The gods of photography, the all time greats, took legendary shots with gear not a quarter as high end as what your average photographer takes for granted today. There is a certain romance about film grain, on a normal sized photo, shot through a regular lens. That feeling, it cannot be quantified or measured in MTF charts.

  • @BrianAndrewParker
    @BrianAndrewParker 2 года назад

    very interesting - can't wait to try this with my mf nikkor lenses from the 60's to better understand why I like them so much!

  • @loosegums
    @loosegums 2 года назад

    T = Heart of Photography

  • @gerardferry3958
    @gerardferry3958 2 года назад

    I never appreciated the difference until I saw it, a Zeiss lens on a sensor without a pass filter really does not need pixel peeping

  • @AnandaGarden
    @AnandaGarden 2 года назад +4

    Wonderful - but I did have to laugh. My ex took a glorious photo of me at sunset while I stood on a shallow reef taking photos of pelicans flying in a lovely symmetrical arc out at sea. I mounted an 11x14 print on my office wall. Oh, and it was taken with an $8 Fujifilm disposable cardboard camera. Meanwhile, all I got with $5,000 worth of Nikon gear was lousy, meaningless pics of some ugly birds. The difference: my ex's focused heart.

  • @kapilbhallafoto
    @kapilbhallafoto 2 года назад

    Very useful Ted, first time getting this info, what does MTF stand for?

  • @aes53
    @aes53 2 года назад +2

    In wine, we call it "bottle variation."

  • @carlosgastelum8251
    @carlosgastelum8251 2 года назад

    Am I having a DejaVu moment or is this a re-upload ? I remember watching this a year or more so ago....

  • @pasapucha
    @pasapucha 2 года назад

    👍️

  • @matthewriley3778
    @matthewriley3778 2 года назад

    Notification gang

  • @markjarrett9400
    @markjarrett9400 2 года назад

    Do you not think that some people have an obsession of lens sharpness over other other properties?

  • @thewildgoose7467
    @thewildgoose7467 2 года назад

    Forget about MTF charts, forget about lens sharpness or megapixels or autofocus points or matrix metering because none of these things will make you a better photographer. Photographers have become obsessed with pixel peeping because the manufacturers want you to be. They don't want you to be happy with your 20mp, they want you to think that you need a 50mp or a 100mp in order to improve your photography. You don't.
    Photography is about 'being there', seeing 'it' and capturing it, and equipment only makes up a tiny fraction of whether a photograph is good or not. If you want to test a lens go out and take some photos with it. If you like the pictures then the lens is good enough. If you don't like the pictures then ask yourself "is it because of the lens or because of me?"
    If the answer is "me" then there's no need to upgrade the lens and the only way to improve is to upgrade "me" by learning more about composition, focal point, lighting, subject matter etc.
    The downside is that takes a bit of work but the upside is you save yourself a lot of cash.

  • @KitagumaIgen
    @KitagumaIgen 2 года назад

    MTF is a very very arrogant way to obfuscate the lens characteristics. The MTF is defined as the "magnitude of the Fourier-transform of the point-spread-function". For the MTF to be mathematically meaningful it is necessary that the PSF is the same over the entire image (shift-invariant) - this is "not always" the case, is it?
    The lens acts as a blurring-filter (even an ideal pin-hole/camera obscure have a diffraction-limited sharpness), and it is a more transparent measure of the focusing to look directly at the point-spread-function in the images, and see hot it varies across the image. One rather ideal source of point-sources are the night-sky with the stars - provided the camera is sensitive enough to capture the stars with short enough exposure-time. Then one can look at how the point-source-stars have blurred out in different parts of the image. This directly reveals the focus of the imaging system. (My background: worked with scientific imaging of aurora borealis, never been a photographer; therefore different starting-point of developing my understanding of this topic.)