Diopters

Diopter is a measure of the power of a lens (or mirror). It is equal to 1/focal length in meters. 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 1/0.5=2 diopters.

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 - -0.75 - Probably don't need glasses

-1.00 - -2.00 - Mild myopia, no differentials needed

-2.00 - -5.00 - Moderate myopia, glasses always needed

-5.00 - -10.00 - High myopia

-10.00+ - Very high myopia. Field of view significantly reduced.