aberrations

Question 1

what is an abberration?

Answer 1

the deviation of an image from perfection

Question 2

what is needed for a lens to produce a clear image?

Answer 2

for each individual point in the object plane to be imaged at a point on the image plane

Question 3

name the two types of aberrations

Answer 3

monochromatic and chromatic

Question 4

what are monochromatic aberrations caused by? when do they occur?

Answer 4

due to geometry of the lens so occur when light is both reflected and refracted and also occurs with monochromatic light

Question 5

what are chromatic aberrations caused by and when do they occur?

Answer 5

caused by dispersion and occur when the lens refractive index varies with it’s wavelength

Question 6

what are the two chromatic aberrations?

Answer 6

transverse and longitudinal aberrations

Question 7

what are the types of non chromatic aberrations

Answer 7

-spherical aberrations
-coma
-oblique astigmatism
-distortions

Question 8

How does the focal length of a lens change with wavelength?

Answer 8

in red light it has a longer wavelngth so optical materials show a smaller refractive index than for the shorter wavelnegth of blue light

Question 9

why does polychromatic light cause chromatic aberrations?

Answer 9

as there is more than one wavelength of light passing through the lens so a number of different coloured images at varying positions and sizes are formed

Question 10

whats the equation for longitudinal chromatic abberations of a lens? (LCA)

Answer 10

LCA = F/V where V is the constringence

Question 11

what two ways is TCA (transverse chromatic aberration) expressed as?

Answer 11

-linear TCA which is the difference in image heights of red and blue colours of light
-the angle (alpha) between the emergent red and blue rays

Question 12

How can you minimise the dispersive effects of a lens?

Answer 12

by using a cemented achromatic doublet by putting a plus and minus lens that have a common curvature together

Question 13

what is required for an achromatic doublet to work?

Answer 13

the chromatic aberrations have to be equal and opposite so
LCA lens 1 = -LCA lens 2w

Question 14

what’s the equation for the power of an achromatic doublet lens?

Answer 14

F1= (FE x V1)/(V1-V2)

Question 15

when is the longitudinal spherical aberration positive? is the lens spherically over or under corrected?

Answer 15

when the focal margin lies to the left of the focal point
the lens is spherically under corrected

Question 16

what lenses will always have SA

Answer 16

lenses with two spherical surfaces

Question 17

how do you best reduce SA in lenses with two spherical surfaces? What problem does this cause?

Answer 17

-restrict the rays to the paraxial region
-problem is this reduces the amount of light passing through the lens and can introduce other problems such as distortion

Question 18

what is coma? when does it occur?

Answer 18

an aberration where the object is not imaged as a circle of list confusion but instead as a comet shaped patch made up of a series of comatic circles where each circle arises from refration of off axis point at a particular zone of the lens
-occurs when objects are off-axis and distant

Question 19

understand the concept of skewed rays

Question 20

give a formula with tangential coma (CT) linking lens aperture and height of paraxial image

Answer 20

CT is directly proportionate to y^2 x h’
where y is the lens aperture and h’ is the heught of the paraxial image

Question 21

give a formula linking tangential coma with saggital coma

Answer 21

CT = 3Cs where Cs is the saggital coma

Question 22

what is sagittal coma?

Answer 22

the displacement from Tp’ of the saggital image point for refraction by the marginal zone

Question 23

What is oblique astigmatism (OA)? what causes OA?

Answer 23

an aberration and visual defect caused by lack of curvature of the cornea

when objects point way off the lens axis, so the image spreads out in a direction along the lens axis

Question 24

understand the concept of oblique astigmatism