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 meter has a power of 3 diopters. The power of a combination of lenses is the sum of their individual powers.