Chapter 8: Aberrations of Optical Systems

Question 1

What are 6 types of optical aberrations in visual systems?

Answer 1

1. chromatic aberration
2. spherical aberration
3. oblique astigmtism
4. coma aberration
5. image distortion
6. curvature of field

Question 2

What is chromatic aberration?

Answer 2

when white light is refracted at an optical surface it is dispersed into its component wavelengths or colours - shorter wavelengths are deviated more on refraction (i.e. blue > red)

therefore a series of coloured images are formed when white light is incident on a spherical lens

Question 3

What is the relationship between the dispersive power of a material and its refractive index?

Answer 3

dispersive power is independent of its refractive index

Question 4

What is used to correct chromatic aberration in lens systems?

Answer 4

achromtic lens systems

Question 5

What are achromatic lens systems?

Answer 5

lenses of varying material combiend so dispersion is neutralised, while overall refractive power is preserved

e.g. combining convex lens (high refractive power, low dispersive power) with concave lens (low refractive power but higher dispersive power), can enutralise aberration

Question 6

What is the total dispersion from the red to the blue image in the human eye due to chromatic aberration?

Answer 6

approx 2.00 D

Question 7

How does the human eye overcome ocular chromatic aberration?

Answer 7

emmetropic eye focuses for the yellow-green (555nm) as this is the peak wavelength of the photopic relative luminosity curve (=between blue and red foci, slightly nearer red)

Question 8

What is a clinical test that makes use of the chromatic aberration of the eye?

Answer 8

Duochrome test

Question 9

How does the duochrome test work to test visual acuity?

Answer 9

patient reports which is clearer; myopic eye will see red more clearly, hypermetropic eye will see green letters more clearly

if green is clearer, patient needs +0.25D until they’re the same

if red is clearer, patient needs -0.25D until the same

Question 10

Why is the duochrome test particularly useful in myopic patients?

Answer 10

experience eye strain if overcorrected + forced to use accommodation for distance vision

should be able to see red letters ore clearly than the green at the end

Question 11

Why doesn’t colour blindness invalidate the duochrome test?

Answer 11

it depends on the position of the iamge with respect to the retina - should ask whether upper or lower rank of letters appears clearer

Question 12

What is spherical aberration?

Answer 12

= prsimatic effect of a spherical lens; rays passing through the periphery of a spherical lens are deviated more than those passing through the paraxial zone

Question 13

What are 4 ways of reducing spherical aberration of a lens?

Answer 13

1. ‘stops’: occluding the periphery of the lens
2. lens form: plano-convex > biconvex
3. aplanatic lens surfaces
4. doublet: 2 lenses cemented

Question 14

How can stops be used to reduce spherical aberration?

Answer 14

they occlude the periphery of the lens so only the paraxial zone is used

Question 15

How can lens form be adjusted to reduce spherical aberration?

Answer 15

plano-convex lenses preferable to biconvex

Question 16

What is an aplanatic lens surface to reduce spherical aberration?

Answer 16

spherical surfaces are abandoned and the lenses are grount with aplanatic surfaces - peripheral curvature less than central curvature

Question 17

What is a doulet when reducing spherical aberration?

Answer 17

principal lens and a weaker lens of different refractive index cemented together

weaker lens should be of opposite power; it also has spherical aberration so will reduce power of the periphery of the principal lens more than the central zone

(usually achromatic as well as spheric)

Question 18

What are 4 factors which reduce spherical aberration in the human eye?

Answer 18

1. Anterior cornea flatter peripherally (aplanatic)
2. Nucleus of lens has higher refractive index than lens cortex - axial zone has greater refractive power
3. Iris acts as a stop
4. Stiles-Crawford effect (type 1): retinal cones more sensitive to light entering the eye paraxially than obliquely

Question 19

What is the impairment of visual acuity when the pupil is dilated due to?

Answer 19

almost entirely due to spherical aberration

Question 20

What is optimum pupil size?

Answer 20

2 - 2.5mm

Question 21

What is the Stiles-Crawford effect?

Answer 21

retinal cones are more sensitive to light which enters the eye paraxilaly than to light which enters obliquely through the peripheral cornea

directional sensitivity limits the visual effects of residual spherical aberration in the eye

Question 22

What is oblique astigmatism?

Answer 22

aberration occuring when rays of light traverse a spherical lens obliquely; when a pencil of light strikes the lens surface obliquely, a toric effect is introduced. Emerging rays form a Sturm’s conoid with 2 line foci

Question 23

What causes oblique astigmatism to occur with spectacle lenses?

Answer 23

when the line of sight is not parallel with the principal axis of the lens; also in patients with restricted eye movement who adopt a compensatory head posture and look obliquely through peripheral portions of spectacle lenses

Question 24

What is the relationship between spectacle lens power and oblique astigmatism?

Answer 24

the higher the spectacle lens power, the greater the unwanted cylindrical power induced by the aberration

Question 25

What is pantoscopic tilt?

Answer 25

in daily life most adults spend most time looking slightly downward from the primary position so loewr borders of the lenses are tilted towards the cheek

Question 26

Which types of spectacle lenses cause worse oblique astigmatism than others?

Answer 26

biconvex and biconcave = much worse than meniscus lenses

Question 27

What are 3 factors making oblique astigmatism minimal in the human eye?

Answer 27

1. Aplanatic curvature of the cornea
2. Spherical surface of the retina means the circle of least confusion of Sturm’s conoid falls on the retina
3. Astigmatic image falls on the peripheral retina which has poor resolving power compared with the retina at the macula so visual appreciation of the astigmatic image is limited

Question 28

What is the definition of coma?

Answer 28

spherical aberration applied to light coming from points not lying on the principal axis

rays passing through the periphery of the lens are deviated more than the central rays and come ot a focus nearer the principal axis, causing unequal magnification of the image formed by different zones of the lens

causing elongated composite image rather than circula (like a coma or comet)

Question 29

What are 2 key ways that coma aberration can be corrected in lenses?

Answer 29

1. Limiting rays to the axial area of the lens
2. Using the principal axis of the lens rather than a subsidiary axis

Question 30

What is the effect of ocular coma aberration and why?

Answer 30

not of practical importance - same reasons as oblique astigmatism (due to aplanatic corneal curvature, spherical retinal surface and Stiles-Crawford)

Question 31

What is image distortion?

Answer 31

when an extended object is viewed through a spherical lens, the edges which are viewed through the peripheral portion are distorted

this is due to increased prismatic effect of the periphery of the lens producing uneven magnification of the object

Question 32

What type of image disortion is produced by a concave lens?

Answer 32

barrel disortion

Question 33

What type of image distortion is produced by a convex lens?

Answer 33

pin cushion distortion

Question 34

What is meant by ‘curvature of field’?

Answer 34

plane object gives rise to a curved image, occurs even when spherical aberration, oblique astigmatism and coma aberrated

Question 35

Why does cuvature of field occur even when spherical aberration, oblique astigmatism and coma have been eliminated?

Answer 35

it is dependent upon the refractive index of the lens material and the curvature of the lens surfaces

Question 36

What compensates for curvature of field in the eye?

Answer 36

curvature of the retina

Question 37

What is Stiles Crawford effect number 2?

Answer 37

there is a shift in the hue and saturation of monochromatic lights depending on the angle of incidence

Question 38

What determines the position of the circle of least confusion from an astigmatic lens?

Answer 38

focal distance of the spherical equivalent of the lens (e.g. +3.00/-2.00x90 would be ½ = 50cm)