Prentice's rule states induced prism is equal to the lens power multiplied by what distance?

• A. Back vertex distance
• B. Axis
• C. Prism diopters
• D. Decentration in centimeters

Answer: (D)

When a lens is decentered from the eye’s optical center, a prismatic effect is produced. Prentice’s rule quantifies this: the induced prism (in prism diopters) equals the lens power (in diopters) multiplied by the decentration distance (in centimeters). In other words, the distance used in the calculation is how far the lens has been decentered from the optical center, measured in centimeters.

For example, a +6.00 D lens decentered by 0.25 cm produces 6.00 × 0.25 = 1.5 prism diopters of induced prism. The other factors listed—back vertex distance, axis, and the unit prism diopters itself—do not represent the distance used in this rule.