Formulas

Question 1

What is the formula to calculate Decentration?

Answer 1

Dec = FPD - PD

FPD is the Frame PD and PD is the patient’s PD.

Question 2

What is the formula for finding the minimum blank size for a lens?

Answer 2

Minimum Blank Size = ED + 2 (Decentration) + 2

ED is the Effective Diameter.

Question 3

How do you calculate Frame PD?

Answer 3

Frame PD = A + DBL

A is the horizontal measurement of the lens, and DBL is the distance between lenses.

Question 4

What does Prentice’s Rule calculate?

Answer 4

Prentice’s Rule calculates the amount of prism induced in lenses or how to move the OC to induce a specific amount of prism.

Question 5

What is the formula for Prentice’s Rule in its original form?

Answer 5

P = H x D / 10
Or P = Hcm x D

P is the prism, H is the height or distance from the OC in mm, and D is the power of the lens.

Question 6

If a lens has a power of -5.00 and is 2mm off, how much prism is induced?

Answer 6

1D of prism

Calculation: P = 2 x 5 / 10 = 1D.

Question 7

What is the formula to find how many millimeters to move a lens to induce a specific amount of prism?

Answer 7

H = P / D x 10

H is the height to move, P is the prism, and D is the power of the lens.

Question 8

How do you calculate the amount of slab off needed for lenses?

Answer 8

Subtract the prism amounts of each eye to find total slab off

Use Prentice’s Rule to find the induced prism for each eye.

Question 9

What is the Nominal Power Formula?

Answer 9

P = F + B

P is the power of the lens, F is the front curve, and B is the back curve.

Question 10

If a lens has a front curve of +6.50 and a back curve of -3.00, what is its power?

Answer 10

+3.50

Calculation: +6.50 + (-3.00) = +3.50.

Question 11

What is the formula to calculate focal length from the power of a lens?

Answer 11

F = 1/D

Focal length is in meters and D is the power of the lens.

Question 12

True or False: Knowing optical formulas can enhance understanding of optical principles.

Answer 12

True

Question 13

Fill in the blank: The effective diameter (ED) is calculated by finding the longest radius from the ______ and multiplying it by two.

Answer 13

geometrical center

Question 14

What is the purpose of knowing optical formulas in the optical industry?

Answer 14

To better understand optical principles and improve job performance.

Question 15

What is the formula for calculating the base curve for plus lenses?

Answer 15

Base curve = spherical equivalent + 6.00D

This formula is used to determine the appropriate base curve for plus lenses.

Question 16

What is the formula for calculating the base curve for minus lenses?

Answer 16

Base curve = ½ spherical equivalent + 6.00D

This formula is applied to find the base curve for minus lenses.

Question 17

If a +2.00D lens is used, what would be the base curve according to Vogel’s formula?

Answer 17

+8.00

This is derived from the formula for plus lenses: +2.00D + 6.00D = +8.00D.

Question 18

True or False: The base curve for a minus lens is calculated using the formula Base curve = spherical equivalent + 6.00D.

Answer 18

False

The correct formula for minus lenses is Base curve = ½ spherical equivalent + 6.00D.