diopter (D) /dī·op″tər/ [Gk, dioptra, optical measuring instrument] , a metric measure of the refractive power of a lens. It is equal to the reciprocal of the focal length of the lens in meters. For example, a lens with a focal length of 0.5 m has a diopter measure of 2.0 (1/0.5) and when prescribed as a corrective lens for the eye should make printed matter most clearly focused when it is held 0.5 m (20 inches) from the eyes.
