Formulas And Calculations

Question 1

Nominal power formula

Answer 1

F1+F2=FT

(D1+D2=DT)(DIOPTERS)

F1: front BC
F2: back power curve
FT: nominal power of the lens

Power is in Diopters

Question 2

Lens form comparisons chart

Answer 2

Question 3

Spherical Equivalent

Answer 3

Sphere + 1/2 Cyl = S.E.

Question 4

Vogels Formula

Answer 4

1) Calculate the S.E.
2) Then,

• if S.E. Is (+) , then : S.E. + 6.00D = BC
• if S.E. Is (-) , then : 1/2 S.E. + 6.00D = BC

BC = F1

Question 5

Distance Between Centers DBC

Answer 5

DBC = A + DBL

Since all decentration calculations will be done monocular in this course, the DBC must be cut in half or divided by a factor of two prior to the completion of the formula.

DBC = (A + DBL)/2

Question 6

Horizontal decentration

Answer 6

(DBC/2) - mono PD

DBC: A+DBL

MUST BE IN MM AND INCLUDE DIRECTION AND EYE.
( in/out , OD/OS)

Question 7

Vertical decentration

Answer 7

Fitting height-(B/2)

B/2: datum line height

MUST BE IN MM AND INCLUDE DIRECTION AND EYE.
(Up/down, OD/OS)

Question 8

Which PD is used for SV distance?

Answer 8

DPD

Question 9

Which PD is used for SV near?

Answer 9

NPD

Question 10

Which PD is used for SV intermediate?

Answer 10

Distance between 40cm and infinity.

Question 11

Which PD is used for bifocals and trifocals?

Answer 11

NPD

Question 12

Which PD is used for PAL’s?

Answer 12

DPD

Question 13

What is the fitting for SV and PAL’s lenses ?

Answer 13

Pupil height

Question 14

What is the fitting for bifocals and trifocals?

Answer 14

Segment height

Question 15

If the Vertical decentration is positive, the lens will move :

Answer 15

Up

Question 16

If the Vertical decentration is negative , the lens will move :

Answer 16

Down

Question 17

Effective Diameter ED

Answer 17

ED= 2x longest radius

May be equal to “A” but never less.

Question 18

Minimum blank size MBS

Answer 18

MBS = ED +2 (monocular decentration)+2mm