The Prentice's Rule formula involves which variables?

• A. Power and Decentration
• B. Pupillary distance and frame size
• C. Power and Index
• D. Frame curvature and size

Answer: (A)

Prentice's Rule shows how a lens’s power and how far the lens is decentered from the optical center interact to produce prism. The two variables involved are the lens power and the amount of decentration. The practical formula is prism diopters equals decentration (in centimeters) times lens power (in diopters). This means higher power or greater decentration creates more induced prism. It’s why precise alignment of the optical center with the pupil matters, especially for strong prescriptions, to avoid unwanted prism or to tailor it for therapy. The other options don’t fit because pupillary distance, frame size, frame curvature, or index aren’t part of Prentice’s Rule—the rule specifically uses power and decentration.