DIOPTER. A unit of measurement used in optics to measure the power of a lens or lens system. The power of a lens is the reciprocal of its focal Length, and when the focal length is ex pressed in metres the power of the lens is in diopters. Thus a lens whose focal length is one metre has a power of one diopter, a lens of focal length 5o cm. (2 metre) a power of 2 diopters, a lens of focal length 2 metres 2 diopter and so on. (See LENS.)