diopter

noun8

1. •
   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.
2. •
   The dioptre adjustment mechanism of a pair of binoculars or of a camera viewfinder.
3. •
   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".
4. •
   (no longer used) Any lens system, such as a telescope.
5. •
   (no longer used) A theodolite or similar surveyor’s angle measuring device.
6. •
   (no longer used) An alidade.
7. •
   (no longer used) A surgical speculum.
8. •
   (no longer used) An instrument for drawing the skull by projections.