Before revision: Illustration of the relationship between optical power and focal length A dioptre (British spelling) or diopter (American spelling) is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dioptre = 1 m−1.
After revision: Illustration of the relationship between optical power and focal length A dpt, symbol dpt, (British spelling) or diopter (American spelling) is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dioptre = 1 m−1.