What is the standard unit used to express the focusing power of a lens?

• A. Diopters
• B. Lumens
• C. Pascals
• D. Watts

Answer: (A)

Focusing power is described using diopters. A diopter is defined as the reciprocal of the focal length in meters (D = 1/f). So a lens with a focal length of 0.5 meters has a power of 2 diopters, and a shorter focal length like 0.25 meters has 4 diopters. Positive diopters indicate converging (convex) lenses, while negative diopters indicate diverging (concave) lenses. This unit directly reflects how strongly a lens bends light, which is why it’s the standard in lens prescriptions and optical design.

Lumens measure light brightness, Pascals measure pressure, and watts measure power or energy transfer—none of these quantify how strongly a lens focuses light, which is why they aren’t used to express focusing power.