CLM - Optics of Contact Lenses I - Week 3

Question 1

What are 5 major differences between contact lenses and spectacle glasses?

Answer 1

Magnification effect
Accommodation and vergence
Field of view
Oblique aberrations
Cosmesis

Question 2

Can the same back vertex power be used for a contact lens as for a spectacle prescription?

Answer 2

Not necessarily, you will need to convert for a contact lenses, especially at high power spectacle lenses

Question 3

Do myopes generally require more or less powered contact lenses than their spectacles? What about hyperopes?

Answer 3

Myopes need lower powered lenses
Hyperopes need higher powered lenses

Question 4

What is the correct way to convert astigmatism power for a contact lens?

Answer 4

Do not consider the cyl power alone, you must decompose the prescription into two sphere lenses, and convert that way

Question 5

How do spectacles affect magnification compared to contact lenses?

Answer 5

Spectacles create large amounts of image magnification/minification for higher prescriptions
Contact lenses create very little of these effects

Question 6

Compare a myope’s view through contact lenses vs spectacles. What about hyperopes.

Answer 6

For myopes, objects appear much smaller with spectacles than contact lenses
For hyperopes, objects appear larger with spectacles than with contact lenses

Question 7

Is better VA possible with contact lenses or spectacles with myopes?

Answer 7

Contact lenses

Question 8

If a hyperope wears contact lenses in place of spectacles, how will this affect their visual environment? What should be considered with a hyperope that is also an early presbyope?

Answer 8

Visual environment appears smaller
Will also make near text appear smaller and possibly harder to see

Question 9

How does accommodative demand change when wearing contact lenses vs spectacles for myopes and hyperopes?

Answer 9

Myopes - contact lenses increase the accommodative demand relative to spectacles
Hyperopes - contact lenses decrease the accommodative demand relative to spectacles

Question 10

What is the accommodative demand like when wearing spectacles for myopes vs hyperopes?

Answer 10

Hyperopes accommodate more than myopes if the spectacles are equivalent power

Question 11

Consider the effect of wearing contact lenses in place of spectacles on accommodative demand. At what power is it clinically relevant? Who else can it be a problem for (2)?

Answer 11

Clinically significant when the prescription is >4.00D
Can be a problem for early presbyopes or those with accommodative problems

Question 12

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

Answer 12

More accommodative demand will mean more accommodative vergence compared with spectacles
For exophores, their phoria will decrease, reducing their need for positive fusional vergence
For esophores, their phoria will increase, increasing their need for negative fusional vergence

Question 13

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

Answer 13

Less accommodative demand will mean less accommodative vergence compared with spectacles
For exophores, their phoria will increase, increasing their need for positive fusional vergence
For esophores, their phoria will decrease, decreasing their need for negative fusional vergence

Question 14

What is the prismatic effect of spectacles 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
-plus lenses induce BO prism, increasing convergence demand

Question 15

What is the prismatic effect of contact lenses when looking at distance vs near? What effect does it have on convergence demand?

Answer 15

No prismatic effect because the lens stays centred on the cornea
Vergence demand is equivalent to being uncorrected

Question 16

Consider a high myopes with high exophoria. What effect would swapping betwen contact lenses and spectacles have on their fusion?

Answer 16

Spectacles - prismatic effect will help them focus more comfortably at near
Contact lenses - with no prismatic effect, they may find near work difficult and could induce asthenopia

Question 17

Consider a high myopes with high esophoria who is accustomed to the prismatic effect of their spectacles. What effect would swapping betwen contact lenses and spectacles have on their fusion?

Answer 17

They might find near work more comfortable while wearing contact lenses due to the decreased demand on negative fusional vergence

Question 18

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

Answer 18

Exophoric shift at near in myopes
Esophoric shift at near in hyperopes

Question 19

Can horizontal prism be incorporated into a contact lens? Explain.

Answer 19

Prism base will always rotate downwards in a contact lenses, so it is not possible.

Question 20

Up to what power vertical prism can be incorporated?

Answer 20

3 diopters of base down prism

Question 21

Do myopes with spectacles have a larger or smaller field of view vs hyperopes with spectacles?

Answer 21

Larger

Question 22

What is field of view only limited by in contact lens wearers?

Answer 22

The degree of eye movements

Question 23

What can occur when viewing through the periphery of the lens of a spectacle? What lenses is this especially apparent in? Does this occur in contact lenses?

Answer 23

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 24

What happens to apparent eye size with minus and plus lenses? Does this occur with contact lenses?

Answer 24

Eyes appear larger with positive prescriptions
Eyes appear smaller with negative prescriptions
This is absent with contact lens wear