Optical power (also referred to as
dioptric power,
refractive 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
-1), which is commonly called the
dioptre.