# diopter

A diopter, also known as a** reciprocal meter**, is a unit used to express
the focal power of optical lenses. One diopter
is the power of a lens that brings parallel light rays to a focus at a point
one meter from the lens, after passing through it. A stronger lens brings
light rays to a focus at a point closer to it than a weaker lens and has
a higher dioptric power.

The power of a converging lens in diopters is a positive number equal to the reciprocal of its focal length in meters. Diverging lenses have negative powers. For example, a diverging lens of focal length 1/3 m has a power of 3 diopters. The power of a combination of lenses is the sum of their individual powers.