dioptric power

Illustration of the relationship between optical power in dioptres and focal length in metres

In optics, optical power (also referred to as dioptric power, refractive power, focal power, focusing power, or convergence power) is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device; high optical power corresponds to short focal length. The SI unit for optical power is the inverse metre (m), which is also called a dioptre (symbol: dpt or D) when used as a unit of optical power.