diopter

A diopter is a unit of optical power used to describe the focusing strength of a lens or refractive surface. It is defined as the reciprocal of the focal length in meters: P = 1/f, with f in meters and P in diopters. One diopter corresponds to a focal length of 1 meter. Positive values denote converging lenses, negative values diverging.

In ophthalmology and optometry, diopters quantify how strongly a lens must bend light to achieve a focused image on the retina. Glasses and contact lenses are prescribed in diopters, with the sign indicating myopia (negative) or hyperopia (positive). Beyond vision correction, diopter values describe the optical power of lenses used in instruments such as cameras, microscopes, and telescopes. Close-up lenses and filter attachments for photography are often rated in diopters to adjust focusing range.

For thin lenses, the combined power of multiple lenses adds: P_total ≈ P1 + P2 + ... . A lens with P = +2.00 D has f ≈ +0.50 m, while P = −3.00 D has f ≈ −0.333 m. In ophthalmic practice, prescriptions may specify spherical (and cylindrical) powers in diopters to correct spherical refractive errors and astigmatism; axis and cylinder values denote orientation of astigmatic correction.

The term diopter originates from the Greek dioptra, reflecting historical measurements of optics; today the diopter remains widely used in practical optics as a standard unit for lens power.