Prentice’s rule: approximate prism magnitude

Example

A meridional power of −5.00 D and 4 mm displacement give |−5 × 0.4| = 2 prism dioptres (Δ). Divide millimetres by ten before multiplying.

Which power to use

For a lens with cylinder, use power in the relevant meridian; do not automatically substitute sphere. The tool receives an already determined meridional power.

Limits

This is an educational approximation without base direction, vector combination or binocular balance. It does not calculate prescribed prism or approve fitting. Off-axis behaviour of a real lens depends on its geometry.

References

• National Academy of Opticianry — Prentice’s Rule
• Technical notes on prism-dioptre and Prentice’s rule (2021)

Related topics

• How to calculate horizontal decentration
• Transposition: another notation for the same prescription