Week 3

Question 1

Entrance Pupil

Answer 1

Virtual image of the pupil or aperture stop when viewed from the object side
The entrance pupil is the size of the physical aperture that would be required to pass the extreme ray to reach the image in the absence of the lens.

Question 2

Exit pupil

Answer 2

Virtual image of the pupil or aperture stop formed by the optical system with viewed from the image side
The exit pupil is 3% larger and 0.07mm behind the iris aperture
the position, shape and size of the exit pupil changes with the direction and distance from which it is being viewed

Question 3

Optimal size of pupil

Answer 3

2-3mm
larger pupils cause aberrations
smaller pupils cause diffraction

Question 4

Optical features of lens

Answer 4

• refractive index is not homogenous (gradient refractive index)
• posterior surface is highly aspheric
• contributes to 25% of the eye’s optical power (15D)
• able to change shape and focus at different distances ( accomodation)

Question 5

Accomodation

Answer 5

• a process by which the refractive index of the crystalline lens increases in order to focus a near object onto the retina
• accomodation occurs by the contraction of ciliary muscles causing zone fibres to relax. This causes the lens capsule to ‘sag’ and move forwards and inwards, forming a more spherical shape ( increase power) ( higher curvature)

Question 6

amplitude of accomodation

Answer 6

the maximum change in refractive power that the crystalline lens can exert during accomodation
amplitude of accomodation decreases with age

Question 7

FPA

Answer 7

the farthest point conjugate to the retina with minimal accomodation exerted

Question 8

NPA

Answer 8

the closest point conjugate to the retina with maximal accomodation exerted

Question 9

presbyopia

Answer 9

age-related refractive disorder in which an individual is unable to focus near objects because of insufficient accomodation due to loss of lens elasticity.
typically affects individuals aged +45

Question 10

Cataracts

Answer 10

yellowing of the lens with age due to increasing absorption of UV radiation and short wavelengths of light ( senile cataract)
Senile cataracts cause reduction in vision as well as contrast sensitivity due to increased light scatter
reduction in the perception of blue and green colours due to the browning of the lens
cataracts can be caused by IR, trauma, pathologies, drug toxicity

Question 11

transmission of light

Answer 11

20% of light incident on the eye is lost in reflection and absorption
25-40% of light is lost in light scatter and becomes stray light
only 40% of light falling on the eye contributes to retinal image

Question 12

stray light

Answer 12

unwanted light that reaches the retina but does not contribute to retinal image
when looking at bright lights it causes loss of visual performance due to stray light known as ‘disability glare’
stray light reduces contrast on an image