Dioptre
= Dioptre
{title2=$1\,\mathrm D=1\,\mathrm{m^{-1}}$}
{wiki}
= Diopter
{synonym}
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.