What is the optical power P of a lens with focal length f?

• A. P = f
• B. P = 1/f (in diopters)
• C. P = 1/do
• D. P = di

Answer: (B)

Power of a lens is defined as the reciprocal of its focal length: P = 1/f, with f measured in meters. This gives P in diopters, since a diopter is 1/m. So a lens with f = 0.5 m has P = 2 diopters, and a lens with f = 2 m has P = 0.5 diopters. The sign of P indicates the lens type: positive for converging lenses and negative for diverging lenses. The other expressions don’t fit this definition or units: P = f would have units of meters, P = 1/do uses object distance rather than focal length, and P = di is not a valid form.