Ophthalmic Optics Formulas Flashcards
Minimum blank size
Total decentration
(A + DBL) - pt PD
Frame PD
A + DBL
ED
Double the distance of the edge of the lens farthest away from geometric center
Frame difference
A size - B size
Exact sag formula
H is half the chord length
Thickness equation for biconvex lens
CT = Sf + ET + Sb
Crown glass
n = 1.523
V = 58
plastic CR-39
n = 1.498
V = 58
Polycarbonate
n = 1.586
V = 30
Trivex
n = 1.53, V=44
Lens clock correction formula
Nominal power
Front surface power + back surface power
F1+F2
Spherical equivalent
SE = sph + cyl/2
Distance target moved on lenso
f is focal length of the standard lens in the lensometer, Fv is Fbvp
Back vertex power
Front vertex power
OC locations of flat-top segs
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)
OC locations for round segs and executives
Round segs -> OC half way through diameter of round seg
Executive seg -> OC is on the line (0mm)
Inset
frame pd - pt pd (divide by 2 if monocular situation)
Seg inset
distance pd - near pd (divide by 2 if monocular situation)
total inset
frame pd - near pd (divide by 2 if monocular situation)
also is seg inset + inset
Image jump
Prism = cF
(distance bw seg OC to dist OC)(add power)
TCA
amount of prism looking through / V