A dioptre, or diopter, is a unit of measurement of the optical
power of a lens or curved mirror, which is equal to the
reciprocal of the focal length measured in metres (that is,
1/metres). For example, a 3 dioptre lens brings parallel rays of
light to focus at 1/3 metre. The same unit is also sometimes
used for other reciprocals of distance, particularly radii of
curvature and the vergence of optical beams. The term was
proposed by French ophthalmologist Felix Monoyer in 1872. Though
the dioptre is based on the SI-metric system it has not been
included in the standard so that there is no international name
or abbreviation for this unit of measurement - within the
international system of units this unit for optical power would
need to be specified explicitly as the inverse metre (m-1).
However most languages have borrowed the original name and some
national standardization bodies like DIN specify a unit name
(dioptrie, dioptria, ..) and derived unit symbol "dpt".
Quantifying a lens in terms of its optical pow