wk3: CLM - Optics of CLs 1 Flashcards
List 5 visual differences between contact lenses and spectacles
Magnification
Accommodation and convergence
FOV
Oblique aberrations
Cosmesis
Should you use the same BVP for a CL as for a spectacle prescription?
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]
What is the lens effectivity equation?
Fcl = Fspec/(1-VdFspec)
When does the power difference between contact lenses and spectacles become significant? (i.e. over 0.25D)
When the magnitude of the ocular refraction is over about +/- 4 diopters
When relevant, how does the power of CLs compare to spectacles in myopes?
myopes require lower powered CLs than their spectacles
When relevant, how does the power of CLs compare to spectacles in hyperopes?
hyperopes require higher powered CLs than their spectacles
How do you calculate lens effectivity for a contact lens with astigmatism?
Do both meridians separately
What is the equation for spectacle magnification?
SM = retinal image size corrected eye/retinal image size uncorrected eye
How does the magnification/minification of CLs differ from spectacle magnification?
CLs create very little magnification or minification
How do objects appear (in terms of size) for a myope with spectacles compared to CLs? How about hyperopes
In spectacles, objects appear much smaller for myopes and larger for hyperopes
How do contact lenses affect accommodative demand relative to spectacles in myopes?
CLs increase the accommodative demand relative to spectacles
How do contact lenses affect accommodative demand relative to spectacles in hyperopes?
CLs decrease the accommodative demand
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
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.
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?
Distance: no prismatic effect
Near
- Minus lenses induce BI prism, reducing convergence demand
- Positive lenses induce BO prism, increasing convergence demand
Explain the prismatic effect from spectacles when looking at distance
no prismatic effect because you are looking through the optical centre
What phoria shift is induced by contact lenses at near in myopes vs hyperopes?
Exophoric shift at near in myopes
Esophoric shift at near in hyperopes
Can horizontlal prism be incorporated into CLs at near? Explain
The prism base will always rotate downwards in a CL, making it impossible to prescribe horizontal prism
What magnitude of vertical prism can be incorporated into a contact lens?
up to 3 diopters of base down prism
Do myopes have a larger or smaller FOV than hyperopes when wearing spectacles?
larger
What is FOV only limited by in contact lens wearers?
the degree of eye movements (typically gives FOV of 100 degrees)
What can occur when looking through the periphery of spectacle lenses? Is this a problem in contact lenses?
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
What happens to apparent eye size of people wearing positive or negative spectacle prescriptions? Does this occur in contact lens wearers?
positive: appears larger
negative: appears smaller
This effect doesn’t occur with contact lenses
