5.1 Off-axis aberrations

Question 1

describe the cause of off-axis aberrations. Draw a diagram to describe this (page 3)

Answer 1

rotation of far point F about the eye’s center of rotation. ideally, lenses focus light upon the far point of the sphere, but this is not possible.

(when looking at the far point of the eye and connecting them, can draw a circle. want light to focus onto it. but since it cannot be done, there are aberrations?)

Question 2

list the important aberrations.

Answer 2

tangential and sagittal power errors (and their combination)

transverse chromatic aberration.

Question 3

equation for tangential power error, sagittal power error, oblique astigmatism and mean oblique error?

Answer 3

deltaT = T - F
delta S = S - F
OA = T - S = deltaT - deltaS
MOE = (T + S)/2 - F = (deltaT - delta S)/2

Question 4

what is the condition for zero oblique astigmatism and zero mean oblique error? draw diagram

Answer 4

1. OA = T - S = 0
2. MOE = 0

page 6.

Question 5

transverse power error and and sagittal power error (vertical and horizontal)?

Answer 5

transverse –> vertical
sagittal –> horizontal (page 4)

Question 6

name the aberrations not important in spectacle lens design

Answer 6

spherical aberration
coma
longitudinal chromatic aberration (diagram, p.8)

Question 7

describe the Tscherning ellipse (how is it made, what does it do)

Answer 7

combining third order aberration theory with thin lenses, tangential and sagittal power errors are quadratic functions of the shape of lens.
when plotted as F2 (or F1) against F for zero oblique astigmatism, we have Tscherning ellipse.

Question 8

the two sections of Tscherning ellipse?

Answer 8

Ostwalt and Wollaston

Question 9

classification and mathematical description of aspheric surfaces. list Q values and what this means. draw diagrams (p.11)

Answer 9

Q = conic constant
Q < -1 hyperboloid
Q = -1 paraboloid
-1<Q<0 ellipsoid with Z axis major axis
Q = 0 sphere
Q > 0 ellipsoid with major axis at right angles to the Z axis

Question 10

why is it that for any power, we can always eliminate an aberration?

Answer 10

aspherizing one surface as conicoid makes third order aberrations cubic functions of shape.

Question 11

list benefits of asphericity

Answer 11

can always eliminate an aberration for any power.

used in moderate power region to improve cosmetics and weight without sacrificing optical performance.

Question 12

nature of Q value of front surface high positive power lens? (+10 to +16D)

(Is it positive or negative?)

Answer 12

negative Q value on front.

Question 13

when dispensing aspheric lenses, must ensure lenses are fitted correctly. what is the relationship between tilt and decentration?

Answer 13

2 degree tilt coincide with 1mm decentration.

Question 14

diagram of balanced tilt and decentration?

Answer 14

p.15

Question 15

check formulas for conicoids pls zc.

Answer 15

page 11 and 12