Prentice's Rule relates prism value to which of the following?

• A. Power times Decentration (cm)
• B. Power plus Decentration (cm)
• C. Decentration divided by Power (cm)
• D. Decentration times Power (mm)

Answer: (A)

Prentice's Rule describes how shifting the lens’ optical center creates a prism effect. The prism amount, in prism diopters, equals the lens power (in diopters) times the horizontal decentration (in centimeters). In other words, Δ = P × d, with d in cm and P in diopters. So the prism diopters you get come from multiplying power by decentration expressed in centimeters. For example, a +6.00 D lens decentered 0.2 cm yields about 1.20 PD of prism (6.00 × 0.2 = 1.20). The rule uses a product, not a sum or a division, and it requires decentration in centimeters (not millimeters).