Prentice's Rule is as follows:

• A. Prism = Power x Decentration (cm)
• B. Prism = Decentration x Power (cm)
• C. Prism = Power + Decentration (cm)
• D. Prism = Decentration - Power (cm)

Answer: (A)

Prentice's Rule shows how decentering a lens creates a prism effect. The prismatic effect, measured in prism diopters, is proportional to both how far the lens is decentered (in centimeters) and the lens power (in diopters). The relationship is Δ = F × d, where Δ is the prism diopters, F is the power in diopters, and d is the decentration in centimeters. Because of this, multiplying the power by the decentration gives the amount of prism produced.

For example, a lens with +6.00 D decentered by 0.2 cm yields Δ = 6.00 × 0.2 = 1.2 Δ. This illustrates why the product is the correct form. The other expressions that use addition or subtraction don’t describe how prism is generated, since prism depends on the product of power and decentration, not their sum or difference.