OPH OPT FORMULAE

Question 1

% reduction in lens thickness due to increase in RI

Answer 1

n1-1 / n2-1 x 100

n1=current lens RI
n2=new lens RI

Question 2

effective power

Answer 2

F/ 1-dF

d is in meters
**follow rx AXIS cross

Question 3

prismatic effect

Answer 3

P= cF (with direction)

c in cm
**use POWER cross

Question 4

MBS

Answer 4

longest axis + decentration (difference of Frame PD & patient’s PD) + 2mm

Question 5

transposition from plus cyl to minus cyl

Answer 5

sph: add up sph and cyl
cyl: change sign to minus
axis: change by 90

Question 6

toric transposition (minus cyl, back surface toric)

Answer 6

Base Curve
___________________
Toric Base Curve / Cross Curve x axis

Base curve (front curve, sphere curve) DS

Toric base curve(DB) weaker/less minus/ more plus

Cross curve (DC) stronger/more minus/ less plus