Chapter 8: Aberrations of Optical Systems Including the Eye

Question 1

Define chromatic aberration.

Answer 1

When white light is refracted, the spectrum is split. Shorter wavelengths (blue) are refracted more than longer wavelengths (red).

NB: Dispersive power is independent of the refractive index

Question 2

How do achromatic lenses work?

Answer 2

Combine different lenses to neutralise the dispersive power of a lens but at the same time, maintain its power.

For example.
Use a convex lens of high refractive power and and lower dispersive power and combine it with a concave lense with low refractive power but higher dispersive power. The overall effect is neurtralisation of the aberration while maintaining an overall convex power.

Question 3

An emmetropic eye focuses which colour most

Answer 3

Yellow-green (slightly closer to red than blue) which is approx 555nm.

Question 4

Describe how the duochrom test works

Answer 4

Patient looks at red green chart.
Red and green are used as their wavelengths straddle the yellow-green light focussed on the retina by an emmitropic eye.
If they see red clearer, they are miopic
If they see green clearer, they are hypermetropic.
If both are the same, they are emmitropic.
NB: test is sensitive to an alteration in refraction of 0.25D or less!

Question 5

When you have given a myopic person glasses, in an ideal situation, which colour should be better visualised on duochrome testing

Answer 5

Red

Question 6

How to reduce spherical aberration of a lens

Answer 6

1. Occlude periphery of the lens so that only axial rays pass through
   2.Switch from biconvex to Plano-concave
2. Use aplanatic lens (edges are less curved that centre).
3. Doublet lens- see diagram

Question 7

How is spherical aberration reduced in the eye.

Answer 7

1. Cornea is aplanatic surface
2. Nucleus of the lens has a higher refractive index than lens cortex
3. Iris blocks non axial rays of light when constricted- optimal is 2-2.5mm
4. Cones and rods are most sensitive to rays are axial- aka stiles-Crawford effect

Question 8

What is oblique astigmatism?

Answer 8

When non axial rays hit a spherical lens. Results in a toric effect which produces strums conoid.
The higher the power the greater the oblique astigmatism

Question 9

What is a pantoscopic tilt

Answer 9

Glasses are slightly tilted towards the cheek bone as most people look slightly downwards. This reduces oblique astigmatism.

Question 10

Which lens shape is least affected by oblique astigmatism

Answer 10

Meniscus lens (worst is biconcave/ biconvex)

Question 11

Why is the effect of oblique astigmatism minimal in the eye

Answer 11

1. Aplanatic surface reduces oblique effect as well as spherical aberration
2. The retina is curved. As a result, the circle of least confusion formed due to oblique astigmatism falls on the retina which is less blurry.
3. Astigmatic image falls on the periphery which has poor resolving power compared to the central retina/ macula

This is also why coma aberration is of minimal importance

Question 12

What is a coma aberration

Answer 12

SPherical aberration of non axial rays. Peripheral are refracted more than central which results in a coma shape

Question 13

When looking at at a grid, what does the image look like when looking through a concave and convex lens respectively.

Answer 13

Prismatic effects of the periphery
Concave looks like a barrel
Convex looks like a pincushion
Particularly annoying for aphakic patients with no lens as they need high power glasses, which have more pronounced prismatic effects (P=FxD)

Question 14

What is Curvature of field and how is it overcome in the eye

Answer 14

One plane images passed through the lens and produce a curved image (even one spherical, aberration, oblique, astigmatism, and coma aberration have been eliminated).
The curvature of the retina compensates for this.