<P> A dioptre (uk), or diopter (us), 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 ⁄ metre . A flat window has an optical power of zero dioptres, and does not converge or diverge light . </P> <P> Dioptres are also sometimes used for other reciprocals of distance, particularly radii of curvature and the vergence of optical beams . The usage was proposed by French ophthalmologist Ferdinand Monoyer in 1872, based on earlier use of the term dioptrice by Johannes Kepler . </P> <P> The main benefit of using optical power rather than focal length is that the lensmaker's equation has the object distance, image distance, and focal length all as reciprocals . A further benefit is that when relatively thin lenses are placed close together their powers approximately add . Thus, a thin 2 - dioptre lens placed close to a thin 0.5 - dioptre lens yields almost the same focal length as a 2.5 - dioptre lens would have . </P>

A is the unit of measurement of a lens refractive power