wk3: CLM - Optics of CLs 1

Question 1

List 5 visual differences between contact lenses and spectacles

Answer 1

Magnification
Accommodation and convergence
FOV
Oblique aberrations
Cosmesis

Question 2

Should you use the same BVP for a CL as for a spectacle prescription?

Answer 2

Not necessarily, because CLs are worn at the corneal plane, we must convert from spectacle refraction to ocular refraction when determining the CL BVP (though this only occurs when power 4 diopters or more) [lens effectivity]

Question 3

What is the lens effectivity equation?

Answer 3

Fcl = Fspec/(1-VdFspec)

Question 4

When does the power difference between contact lenses and spectacles become significant? (i.e. over 0.25D)

Answer 4

When the magnitude of the ocular refraction is over about +/- 4 diopters

Question 5

When relevant, how does the power of CLs compare to spectacles in myopes?

Answer 5

myopes require lower powered CLs than their spectacles

Question 6

When relevant, how does the power of CLs compare to spectacles in hyperopes?

Answer 6

hyperopes require higher powered CLs than their spectacles

Question 7

How do you calculate lens effectivity for a contact lens with astigmatism?

Answer 7

Do both meridians separately

Question 8

What is the equation for spectacle magnification?

Answer 8

SM = retinal image size corrected eye/retinal image size uncorrected eye

Question 9

How does the magnification/minification of CLs differ from spectacle magnification?

Answer 9

CLs create very little magnification or minification

Question 10

How do objects appear (in terms of size) for a myope with spectacles compared to CLs? How about hyperopes

Answer 10

In spectacles, objects appear much smaller for myopes and larger for hyperopes

Question 11

How do contact lenses affect accommodative demand relative to spectacles in myopes?

Answer 11

CLs increase the accommodative demand relative to spectacles

Question 12

How do contact lenses affect accommodative demand relative to spectacles in hyperopes?

Answer 12

CLs decrease the accommodative demand

Question 13

Consider the effect of a myope wearing contact lenses in place of spectacles. What would happen to the vergence demand? Describe the effect on exophores and esophores

Answer 13

More accommodative demand will mean more accommodative vergence compared to spectacles. For exophores, their exophoria will decrease, reducing their need for positive fusional vergence. For esophores, their esophoria will increase, increasing their need for negative fusional vergence.

Question 14

What is the prismatic effect when looking at distance vs near (for both plus and minus lenses)? What effect does it have on convergence demand?

Answer 14

Distance: no prismatic effect
Near
- Minus lenses induce BI prism, reducing convergence demand
- Positive lenses induce BO prism, increasing convergence demand

Question 15

Explain the prismatic effect from spectacles when looking at distance

Answer 15

no prismatic effect because you are looking through the optical centre

Question 16

What phoria shift is induced by contact lenses at near in myopes vs hyperopes?

Answer 16

Exophoric shift at near in myopes

Esophoric shift at near in hyperopes

Question 17

Can horizontlal prism be incorporated into CLs at near? Explain

Answer 17

The prism base will always rotate downwards in a CL, making it impossible to prescribe horizontal prism

Question 18

What magnitude of vertical prism can be incorporated into a contact lens?

Answer 18

up to 3 diopters of base down prism

Question 19

Do myopes have a larger or smaller FOV than hyperopes when wearing spectacles?

Answer 19

larger

Question 20

What is FOV only limited by in contact lens wearers?

Answer 20

the degree of eye movements (typically gives FOV of 100 degrees)

Question 21

What can occur when looking through the periphery of spectacle lenses? Is this a problem in contact lenses?

Answer 21

excursion eye movements result in viewing through the periphery of the lens which may induce significant oblique aberrations (especially in high powered lenses). This is not a problem in contact lenses, as the patient is always viewing through the optical centre of the lens

Question 22

What happens to apparent eye size of people wearing positive or negative spectacle prescriptions? Does this occur in contact lens wearers?

Answer 22

positive: appears larger
negative: appears smaller
This effect doesn’t occur with contact lenses