8. Aberrations of Optical Systems

Question 1

Chromatic aberration

Answer 1

when white light is refractive at an iptical interface it is dispersed into its component wavelengths or colours

Question 2

in chromatic aberration, the shorter the wavelenth

Answer 2

the more it is deivated

Question 3

chromaitc aberration is not desierable - T/F

Answer 3

True
lenses used in instruments are designed to eleminate it

Question 4

Dispersive power of a material is not linked to its refractive index - T/F

Answer 4

True
materials can have a high refractive index but low dispersive power

Question 5

achromatic lens systemts

Answer 5

composed of elements of varying material combine so that the dispersion is neutralised while the overall refractive power is preserved

Question 6

what were the earliest achromatic lenses made fromt

Answer 6

flint and crown glass

Question 7

refraction by the eye is subject to chromatic aberration - T/F

Answer 7

True

Question 8

what is the total dispersion from red to the blue image

Answer 8

approx 2.00D

Question 9

where does the emmetropic eye focus

Answer 9

on yellow green 555nm

Question 10

why does the emmetropic eye focus of yellow green

Answer 10

as this is the peak wavelenght of the phtopic relative luminosity curve

Question 11

where does the focus lie between

Answer 11

between the blue and red foci, slight nearer to the red end

Question 12

what is the duochrome test and how is it perfromed

Answer 12

assess for chromatic aberration to check refraction in myopic patients

Upper rank mounted on red and lower on green
Patients views the letweer by red and green light respectivtly and are asked to say which they see clearer

Question 13

why is red and green used in the duochrome test

Answer 13

as their wavelength foci straddle the yellow greed by equal amoiunts - 0.4D either side

Question 14

how sensitive is the duochrome test

Answer 14

can pick up refraction of 0.25D of less

Question 15

a myopic eye sees which more clearly

Answer 15

green

Question 16

a hypermetropic eye sees which more clearly

Answer 16

red

Question 17

at the end of myopic refraction, which should they see more clearly

Answer 17

RED > green

If overcorrected, made hypermetropic and using accommodation for distance

Question 18

Spherical aberration

Answer 18

Prismatic effect – rays passing at periphery are deviated more

Question 19

ways to reduce spherical aberration

Answer 19

o Use of stops
o Adjusting the lens
o Doublet lens

Question 20

how do stops help to reduce spherical aberration

Answer 20

only the paraxial zone is used

Question 21

how does changing the lens help to reduce spherical aberration

Answer 21

plano-convex > biconvex

Question 22

how do doublet lenses help to reduce spherical aberration

Answer 22

principal lens and a weaker lens of opposite power and different refractive index

Question 23

ways ocular spherical aberration is reduced

Answer 23

o Anterior corneal surface is aspheric (aplanatic)
o Axial sone of the lens (nucleus) greater refractive power than periphery (cortex)
o Iris acts as a stop – optimum size is 2-2.5mm
o Stile-crawford effect – retinal cones more sensitive to light entering the eye paraxially

Question 24

oblique astigmatisim

Answer 24

o Occurs when light enters the eye obliquely i.e. no parallel to the principal axis  toric effect  sturm’s conoid

Question 25

higher glasses prescriptions produce

Answer 25

higher amounts of oblique astigmatism
 Worse for biconvex and biconcave lenes

Question 26

pantoscoptic tilt

Answer 26

 Glasses are made with the lower borders of the lenses tilted towards the cheek to reduce obliquity of the reading potion

Question 27

best form lenses

Answer 27

reducing both spherical and oblique aberrations, usually meniscus form

Question 28

ocular oblique astigmatism is reduced by

Answer 28

o Aplanatic curvature of the cornea
o Retinal is a spherical surface - circle of least confusion of the strum’s conoid falls on the retina
o Astigmatic image falls on the peripheral retina – visual appreciation of astigmatic image is limited

Question 29

coma aberation

Answer 29

spherical aberration for light coming from points not lying on the principal axis
o Rays passing through the periphery of the lens are deviated more
o This produces unequal magnification
o Not optically significant due to reasons for ocular oblique astigmatisim

Question 30

low order aberrations

Answer 30

myopia, hyperopia, astigmatism

Question 31

image distortion caused by a convex lens

Answer 31

pin-cushion

Question 32

image distortion caused by a concave lens

Answer 32

barrel distortion

Question 33

curvature of field

Answer 33

o Plane objects give rise to a curved image
o Related to – refractive index of lens and cruvate of the lens surface
o Curvature of the retina compensates for curvature of field