MCQ's in Optometry Part 5

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Explaination
1. b. 167
Cylinder axis: 77
Cylinder power is 90 degrees away: 77 + 90 = 167
2. d. 180
Cylinder power required at axis: 90
Cylinder should be oriented 90 degrees away: 90 + 90 = 180
3. a. -1 D
The total cylindrical power of a combination of cylindrical lenses, held with
their axes parallel, is the algebraic (not arithmetic) sum of each power:
Total cylinder power with axes parallel: (+ 1) + (-2) = + 1 - 2 = -1 D
4. c. 0.5 D
The cylindrical lenses in any Jackson Cross-Cylinder (manual or on a refractor)
are of opposite signs and oriented with their axes at 90 degrees to each
other. Cylindrical lenses oriented in such a way will manifest both powers, and
the total cylindrical power of a combination of such lenses is the arithmetic (not
algebraic) sum of each power:
Total power of cylindrical lenses of opposite signs and held with their axes at
90 degrees to each other: (0.25) + (0.25) = 0.5 D
Similarly, Jackson Cross-Cylinders with powers of ±0.50 D, ±0.75 D, and ±1 D
will have total cylindrical powers of, respectively, 1 D, 1.5 D, and 2 D.
5. c. +2 D
(Please see explanation for Question 4, above, for details.)
Total power of cylindrical lenses of opposite signs and held with their axes at
90 degrees to each other: (+1) + (+1) = 1 + 1 = 2 D
6. b. have the same power in all meridians
Have different powers along various meridians
(a.) is incorrect since that is a characteristic of spherocylindrical lenses.
Only converge light
(c.) is incorrect since that is a characteristic of plus lenses, both spherical and cylindrical.
Only diverge light
(d.) is incorrect since that is a characteristic of minus
lenses, both spherical and cylindrical.
7. c. lenses that reduce image sizes
Patients with excessive plus vergence are myopic, meaning refractive error
is corrected using minus lenses. Reducing image size is an important characteristic
of minus lenses. Lenses that magnify images (a.) is incorrect since that is a characteristic of
plus lenses. PL Sph (b.) is incorrect since myopic patients need refractive correction.
A +5 D lens (d.) is incorrect since a myope using plus lenses will become even
more myopic.
8 d. +4.50 -1.50 x 180
Plus cylinder to minus cylinder:
Plus cylinder: +3.00 +1.50 x 90
sphere cylinder cylinder
power axis Algebraically add the sphere and cylinder: (+3) + (+1.50) = +4.50
New sphere power: +4.50
Copy the full cylinder power: 1.50
Change the sign of the cylinder power: -1.50
Change the axis by 90 (180 is the maximum): (90 + 90 = 180)
The transposed Rx is:
Minus cylinder: +4.50 -1.50 x 180
9. a. +8.00 -4.00 x 90
Plus cylinder to minus cylinder:
Plus cylinder: +4.00 +4.00 x 180
sphere cylinder cylinder power axis Algebraically add the sphere and cylinder: (+4) + (+4) = +8
New sphere power: +8.00
Copy the full cylinder power: 4.00
Change the sign of the cylinder power: -4.00
Change the axis by 90 (180 is the maximum): (180 - 90 = 90)
The transposed Rx is:
Minus cylinder: +8.00 -4.00 x 90
10. b. two refracting surfaces
Spherocylindrical lenses are combinations of spherical and cylindrical lenses.
Such lenses have two refracting surfaces of different powers, which together
produce the refraction characteristic of a spherocylindrical lens.
One plano surface (a.) and one refracting surface (d.) are incorrect since
these are characteristics of cylindrical lenses.
Two plano surfaces (c.) is incorrect since this is a characteristic of lenses with
no power.
11. a. spherocylindrical lenses
Compound astigmatism must be corrected along two meridians with different
powers. A lens with different powers along two meridians is a spherocylindrical
lens.
Spherical lenses (b.) is incorrect since these lenses have the same power in
all meridians, and correct myopia and hyperopia.
Cylinder lenses alone (c.) is incorrect since such lenses correct simple astigmatism.
Plano lenses (d.) is incorrect since these lenses do not have any power.
12. d. mixed astigmatism
Plus cylinder form: -2.00 +4.00 x 87
Minus cylinder form: +2.00 -4.00 x 177
Plus power is excessive along one axis, requiring a minus lens (-2.00 at axis
87), and inadequate 90 degrees away, requiring a plus lens at that axis (+2.00
at axis 177). Such variations in plus power of the eye are characteristic of mixed
astigmatism. In lens correction for mixed astigmatism, the cylinder power is
greater than the sphere power, and of opposite sign.
Compound hyperopic astigmatism (a.) is incorrect since the sphere powers
in plus and minus cylinder forms should be plus.
Compound myopic astigmatism (b.) is incorrect since the sphere powers in
plus and minus cylinder forms should be minus.
Simple astigmatism (c.)
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