A unit of length used to express the focal length of lenses; currently, chiefly of eyeglasses, magnifying glasses and close-up attachments for photography. The diopter value is the reciprocal of the focal length in meters: a 3-diopter lens has a focal length of ¹/₃ meter. The greater the number, the greater the magnification.
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Last revised: 23 December 2007.