<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> <P> Though the dioptre is based on the SI - metric system it has not been included in the standard so that there is no international name or abbreviation for this unit of measurement--within the international system of units, this unit for optical power would need to be specified explicitly as the inverse metre (m). However most languages have borrowed the original name and some national standardization bodies like DIN specify a unit name (dioptrie, dioptria, etc .) and derived unit symbol "dpt". </P> <P> The fact that optical powers are approximately additive enables an eye care professional to prescribe corrective lenses as a simple correction to the eye's optical power, rather than doing a detailed analysis of the entire optical system (the eye and the lens). Optical power can also be used to adjust a basic prescription for reading . Thus an eye care professional, having determined that a myopic (nearsighted) person requires a basic correction of, say, − 2 dioptres to restore normal distance vision, might then make a further prescription of' add 1' for reading, to make up for lack of accommodation (ability to alter focus). This is the same as saying that − 1 dioptre lenses are prescribed for reading . </P>

Si unit of power of a lens is