Diopters

Diopter is a measure of the power of a lens (or mirror) and is equal to the reciprocal of focal length in meters. The unit symbol for diopters is dpt, D, or m^-1.

$D={\frac {1}{f}}$

In EM we refer to the cm measurement to calculate diopters needed to correct refraction of the eye. If you can see 50cm clearly your diopters will be ${\frac {1}{0.50}}=2dpt$. Lenses in series add their powers : if you're wearing -2 diopter contacts (adjusted for glasses strength) and put +1 diopter reading glasses over the contacts you're in effect wearing -1 diopters. A lens with a negative diopter sign compensates for nearsightedness while a lens with a positive diopter sign compensates for farsightedness.

Approximate categorizations of myopia by spherical lens power:

0.00 to -0.75 dpt	Probably don't need glasses
-1.00 to -2.00 dpt	Mild myopia, no differentials needed
-2.00 to -5.00 dpt	Moderate myopia, glasses always needed
-5.00 to -10.00 dpt	High myopia
-10.00+ dpt	Very high myopia. Field of view significantly reduced.