Diopters

The diopter (symbol D) is a unit of optical power equal to the reciprocal of the focal length in meters. It is defined as D = 1/f, where f is the focal length in meters. The diopter is not an SI base or derived unit, but it is accepted for use with SI and is widely used in optics, ophthalmology, optometry, photography, and vision science.

The sign of the diopter indicates the type of lens: positive diopters correspond to converging (convex) lenses and are used to correct farsightedness (hyperopia) or for magnifying applications; negative diopters correspond to diverging (concave) lenses and are used to correct nearsightedness (myopia). The focal length of a lens with power D is f = 1/D. For example, a 2.00 D lens has a focal length of 0.50 meters, while a −3.00 D lens has a focal length of −0.333 meters.

In practical use, eyeglasses and contact lenses are prescribed in diopters to shift the eye’s focal point so that images focus on the retina. Higher diopter values indicate stronger corrective power. The diopter concept also applies to other optics, such as camera lenses and magnifying glasses, where greater diopter power denotes stronger focusing ability.

Astigmatism corrections may involve cylinder power expressed in diopters (D) and an axis angle. Cylindrical diopters describe additional refractive strength in a specific meridian, and are combined with spherical diopters to form a complete prescription.

Diopter is a derived unit of reciprocal length (m^-1) and serves as the common language for describing lens strength across optical disciplines.