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.

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 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 Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \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.