Dioptric Adjustment, often known as a diopter (or Dioptre) is an optical control on the viewfinder of a camera that allows someone to adjust the viewfinder's magnification to their vision, removing the need to wear eyeglasses when looking through it. Diopter adjustments cannot compensate for all vision problems, only near and far sightedness can be compensated.
Technically, Dioptre is a unit for measuring the power of lenses, equal to the reciprocal of the focal length (in metres). Positive numbers are convex lenses, concave lenses have negative dioptric values. The unit is used by opticians to measure the strength of eye-glass lenses (hence the usage of the term for viewfinder adjustment), and by photographers to measure the strength of (for example) auxiliary close-up lenses. Close up lenses fitted to the filter thread on the front of a lens may typically have dioptre values of between +1 and +3.