 Dioptre

A dioptre, also spelled diopter, is the unit of optical power equal to one reciprocal metre. In $\Phi =1/f$, convert focal length to metres before taking the reciprocal: $f=25\mathrm{cm}=0.25\mathrm{m}$ gives $\Phi =4\mathrm{D}$, not $0.04\mathrm{D}$. A dioptre measures focusing strength, distinct from the transverse magnification of an image.

A dioptre (or diopter in American English) is a unit of measurement that expresses the optical power of a lens or curved mirror. The optical power is defined as the inverse of the focal length (in meters) of the lens or mirror. The formula for calculating the optical power in dioptres (D) is: \[ D = \frac{1}{f} \] where \( f \) is the focal length in meters.