3c. Image Quality on the Retina

Question 1

Pupil Constriction

• Muscle
• Innervation

Answer 1

Circumferentially arranged fibres of the iris sphincter muscle under parasympathetic innervation

Question 2

Pupil Constriction

- Pathway

Answer 2

• Cranial nerve II (optic)
• Pretectum of midbrain
• Bilateral projection to Edinger-Westphal nucleus
• Preganglionic parasympathetic neurones to ciliary ganglion
• Postganglionic parasympathetic neurones to iris sphincter muscles

Question 3

Pupil Constriction

- Inhibition

Answer 3

Atropine

Question 4

Pupil Dilation

• Muscle
• Innervation

Answer 4

Radially arranged fibres of the iris dilator muscle under sympathetic innervation

Question 5

Focussing Light

- 3 ways

Answer 5

• Convex surfaces
• Varying refractive indices
• Accommodation

Question 6

Accommodation

- Process

Answer 6

1. Parasympathetic stimulation of the circular ciliary muscle via cranial nerve III (oculomotor)
2. Contraction of ciliary body
3. Relaxation of zonules of Zinn
4. Lens becomes more spherical with a higher refractive index
5. Focusses light from nearby objects

Question 7

Accommodation

- Different Types

Answer 7

Cats:
Cats move the lens backwards and forwards without changing its shape to accommodate

Chickens:
Change the shape of the cornea as well as the lens to accommodate

Mice:
Don’t accommodate at all

Question 8

Presbyopia

Answer 8

Decreasing lens elasticity with increasing age, leading to a reduction in accommodating power

Question 9

Nyquist Limit

Answer 9

The requirement for photoreceptor spacing to be at least twice as fine as the width of the point-spread function to differentiate 2 separate objects as distinct.

Question 10

3 Factors Limiting Spatial Resolution

Answer 10

• Diffraction
• Lens abberations
• Refractive defects

Question 11

Diffraction

Answer 11

Waves form as light passes through a small aperture, forming a blurred circle known as the point-spread function on the retina

Question 12

Reducing Diffraction

- 2 Methods

Answer 12

• Make aperture larger

- Use shorter wavelength light

Question 13

Point Spread Function

Answer 13

3D diffraction pattern of light emitted from an indefinitely small point source that is represented on the retina

Caused by:

• Diffraction
• Lens Aberrations

Question 14

Lens Aberrations

• Description
• 3 Types

Answer 14

Imperfections in the eye that degrade the image

3 types:

• Spherical aberration
• Chromatic aberration
• Glare

Question 15

Spherical Aberration

- Description

Answer 15

Spherical lens brings peripheral light rays into focus closer to the lens than central light rays, meaning that peripheral light rays will have focussed before reaching the retinal, spreading out again and leaving a blurry fringe around the image in focus

Question 16

Spherical Aberration

- Prevention

Answer 16

• Flattening the outer margins of the cornea

- Changing the refractive index of the lens centre so it is stronger than in the periphery

Question 17

Chromatic Aberration

- Description

Answer 17

The refractive index of ocular material varies as a function of wavelength, so different wavelengths of light focus at different focal lengths.

Blue light is refracted more strongly than red and yellow wavelengths so focuses much closer to the lens

The human eye is well adapted to focus green light:

• Blue light will have focussed before the retina and spread out again
• Red light will not have focussed as it passes through the retina

Question 18

Glare

- Description

Answer 18

Scattered particles in the ocular material causes light scattering, giving rise to glare that reduces the contrast of the retinal image

Question 19

Diffraction vs Aberration

Answer 19

Small pupil diameter:

• High diffraction
• Low aberration

Large pupil diameter:

• Low diffraction
• High aberration

Question 20

Ideal Pupil Size

- Human

Answer 20

2mm

• Aberration contributes a small amount to point-spread function
• Central region is similar to the diffraction limit

Question 21

Refractive Errors

- 2 Examples

Answer 21

• Myopia

- Hypermetropia

Question 22

Refractive Errors

- Description

Answer 22

When light is brought into sharp focus on the retina = emmetropic

Refractive errors are where light is not brought into sharp focus on the retina = ametropic

Question 23

Myopia

- Description

Answer 23

Short-sightedness

• Optics are too strong
• Eyeball is too long (more common)

Rays come into focus before the retina so begin to diverge again, forming a blurry image for distant objects

Question 24

Myopia

- Prevalence

Answer 24

20% of the human population

Question 25

Myopia

- Correction

Answer 25

Diverging lens to spread rays so that they appear to come from the eye’s appropriately close far point and form a perfect image on the retina

Question 26

Hypermetropia

- Description

Answer 26

Long-sightedness

• Optics are not strong enough

Light focusses after the retina, so even when the object is at infinite distance and light rays are parallel objects seem blurry

Question 27

Hypermetropia

- Prevalence

Answer 27

30% of the human population

Question 28

Hypermetropia

- Correction

Answer 28

Converging lens to add extra optic power and allow light rays to come into perfect focus on the retina

Question 29

Stiles-Crawford Effect

Answer 29

The ellipsoids of cones act as waveguides, reducing cone absorption of oblique light rays to yield a sharper retinal image.

Rods absorb oblique light rays.