A dioptre is the basic unit of optical power for lens systems in ophthalmology and optometry, defined as the reciprocal of the focal length of a lens expressed in metres (1 D = 1/f[m]). A lens with a power of 1 dioptre has a focal length of 1 metre, a lens with a power of 2 dioptres has a focal length of 0.5 metres, and a lens with a power of -1 dioptre (negative) has a focal length of -1 metre, which corresponds to a diverging lens. A dioptre is a measure of the ability of a lens or optical system to converge (focus) or diverge (disperse) light rays, with positive values (+) denote converging lenses used to correct hyperopia and presbyopia, whilst negative (-) values denote diverging lenses used to correct myopia.
In clinical practice, dioptres are used to determine refractive errors eye (sphere: the value of the spherical correction power; cylinder: the value of the cylindrical power in astigmatism) and for specifying the power of spectacle lenses, contact lenses and intraocular lenses. The total optical power of the human eye is approximately 60 dioptres (of which about 43 D is attributable to the cornea and about 17 D to the lens), although this value is subject to physiological changes during accommodation. In an ophthalmic prescription, dioptre values are specified with a certain degree of precision – usually in increments of 0.25 D for spherical and cylindrical powers in spectacle corrections, whilst for intraocular lenses, increments of 0.5 D or smaller are available, depending on the manufacturer and model. The refractive power of an optical system comprising several lenses is approximately the algebraic sum of the powers of the individual lenses (when they are placed close together), which allows for a simple calculation of the total optical correction, although precise calculations must take into account the distances between the lenses and the vertex distance (the distance between the spectacle lens and the cornea).