Ophthalmic Optics Formulas

Question 1

Minimum blank size

Answer 1

Question 2

Total decentration

Answer 2

(A + DBL) - pt PD

Question 3

Frame PD

Answer 3

A + DBL

Question 4

ED

Answer 4

Double the distance of the edge of the lens farthest away from geometric center

Question 5

Frame difference

Answer 5

A size - B size

Question 6

Exact sag formula

Answer 6

H is half the chord length

Question 7

Thickness equation for biconvex lens

Answer 7

CT = Sf + ET + Sb

Question 8

Crown glass

Answer 8

n = 1.523
V = 58

Question 9

plastic CR-39

Answer 9

n = 1.498
V = 58

Question 10

Polycarbonate

Answer 10

n = 1.586
V = 30

Question 11

Trivex

Answer 11

n = 1.53, V=44

Question 12

Lens clock correction formula

Answer 12

Question 13

Nominal power

Answer 13

Front surface power + back surface power
F1+F2

Question 14

Spherical equivalent

Answer 14

SE = sph + cyl/2

Question 15

Distance target moved on lenso

Answer 15

f is focal length of the standard lens in the lensometer, Fv is Fbvp

Question 16

Back vertex power

Answer 16

Question 17

Front vertex power

Answer 17

Question 18

OC locations of flat-top segs

Answer 18

For 22, 25, 28mm -> OC is 5mm below top of seg
For 35mm -> OC is 4.5mm below top of seg
For 40, 45mm -> OC is on the line (0mm)

Question 19

OC locations for round segs and executives

Answer 19

Round segs -> OC half way through diameter of round seg
Executive seg -> OC is on the line (0mm)

Question 20

Inset

Answer 20

frame pd - pt pd (divide by 2 if monocular situation)

Question 21

Seg inset

Answer 21

distance pd - near pd (divide by 2 if monocular situation)

Question 22

total inset

Answer 22

frame pd - near pd (divide by 2 if monocular situation)

also is seg inset + inset

Question 23

Image jump

Answer 23

Prism = cF
(distance bw seg OC to dist OC)(add power)

Question 24

TCA

Answer 24

amount of prism looking through / V

Question 25

LCA

Answer 25

Power of lens / V

Question 26

power error

Answer 26

(tangential error + sagittal error) /2

Question 27

Effective power

Answer 27

left to right direction is positive

Question 28

prism diopters

Answer 28

= cm/m (image jump over distance)

Question 29

Angle of deviation (approx)

Answer 29

D = A (n-1)

Question 30

Prism based on thickness of base and apex

Answer 30

Prism = 100g(n-1) / d

d is distance bw apex and base
g is thickness difference bw apex and base
(match units)

Question 31

One degree of deviation angle for a prism is how many prism diopters?

Answer 31

1.75 pd

Question 32

Prentice rule

Answer 32

Question 33

Amount of BD necessary in progressive lens

Answer 33

Add power x 2/3

Question 34

Spec mag

Answer 34

Question 35

Reflection formula

Answer 35

I is intensity of the light incident on the surface

Question 36

Index relationship with reflection/transmittance

Answer 36

Increased n = increased reflections = decreased transmittance
lenses seem dimmer to look through

Question 37

Ultimate transmission

Answer 37

T(ultimate) = (T1)(T2)(T3) …

Question 38

Ideal AR coat index formula

Answer 38

Question 39

Lens maker equation

Answer 39

P = (n2 - n1) / r

Question 40

Correcting vertical imbalance

Answer 40

Clinical methods - Slab off (BUMM, base up more minus), multiple specs, CLs, Fresnel prisms
Academic methods - dissimilar segs, compensated R segs

Question 41

Prentice rule for image jump in ADDs

Answer 41

Prism = dP , where d is the distance from the top of seg to near OC, and P is the power of the ADD only