diopter

A unit of measure of the power of a lens or mirror, equal to the reciprocal of its focal length in meters. Myopia is diagnosed and measured in diopters.
The dioptre adjustment mechanism of a pair of binoculars or of a camera viewfinder.
(dated) A lens element attached to the front of a photographic lens to shorten the lens's minimum focus distance; also called a "close-up filter".
(obsolete) Any lens system, such as a telescope.
(obsolete) A theodolite or similar surveyor’s angle measuring device.
(obsolete) An alidade.
(obsolete) A surgical speculum.
(obsolete) An instrument for drawing the skull by projections.