Diopter (D)

A unit of measure of the refractive power of a lens or of an eye, where the dioptric power is the inverse (reciprocal) of the focal length of the lens or of the eye’s optical system in meters; a +1.00 D convex lens focuses light, from an infinite source, 1 ÷ 1.00 D = 1 meter away from the lens (and corrects an eye which is 1.00 D hyperopic); a +2.00 D lens focuses infinite light 1 ÷ 2.00 D = ½ meter away (and corrects an eye which is 2.00 D hyperopic); a +0.25 D lens focuses infinite light 1 ÷ 0.25 D = 4 meters away (and corrects an eye which is 0.25 D hyperopic); etc.