A dioptre is a unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres (that is, 1/metres). It is thus a unit of reciprocal length. For example, a 3-dioptre lens brings parallel rays of light to focus at 1⁄3 metre. Optical power of the human eye is 58 D.
Diopter value in the spherical contact lenses is the same in different places, as opposed to toric contact lenses (a combination of community and cylinder), bifocal contact lenses and multifocal contact lenses.