Formulas And Definitions

Question 0

What is minimum blank size? How do you calculate it?

Answer 0

Blanks are big rounded lens( uncut lens)
A=horizontal DBL= short distance between 2 lens ED=effective diameter of lens PD= pupillary distance
A+DBL+ED-PD=Minimum blanks

Question 1

What is Decentration? How do you calculate it?

Answer 1

Decentration is the difference between the geometric center of the lens and the optical center of the lens expressed in mill-meters
A = horizontal B= vertical DBL= shortest between 2 lens PD=pupillary distance
A +DBL - PD= total Decentration divide by 2 for per lens

Question 2

What is the Prentice’s rule for? What is the formula?

Answer 2

Prentice’s rule is the formula used to calculate the amount of prism induced in a lens when the optical center does not align with the wears pupil.

Prism power = Decentration in centimeters X Focal length( power of lens, diopter)