Optical Calculators

Induced Prism Calculator

Find the prism a patient gets when they look through a point other than the lens's optical center. Enter the Rx and the decentration, and you'll get the vertical and horizontal prism.

Induced prism comes from decentration: a misaligned frame, an off-center lens, or simply looking through the bottom of a bifocal. The calculator uses Prentice's Rule (prism = decentration in cm × power), so stronger lenses and bigger offsets produce more prism.

Inputs:

  • Lens Rx (sphere, cylinder, axis): the calculator works out the power along the vertical and horizontal meridians.
  • Decentration (mm): how far up or down and in or out the pupil sits from the optical center.

Results: vertical prism (base up or down) and horizontal prism (base in or out).

Use it to check finished glasses against ANSI tolerances and catch errors before they cause headaches, pulling or double vision. To compare both eyes at the reading level, use the Vertical Imbalance Calculator.

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