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.

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• 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.

Gap and ratio

Comparisons between two diopters is typically expressed using one of these terms:

• diopter gap (or diopter difference): absolute difference in diopters between the values of the two eyes
• diopter ratio: ratio of the diopters in one eye over the other one (right eye / left eye)

For example,the following correction:

OD: -1.5 SPH / -1.5 CYL
OS: -1.0 SPH / -2.0 CYL

can be expressed as a 0.5 dpt gap in both SPH and CYL, a 1.5 ratio in SPH and a 0.75 ratio in CYL:

|(-1.5 dpt) - (-1.0 dpt)| = 0.5 dpt
|(-1.5 dpt) - (-2.0 dpt)| = 0.5 dpt
(-1.5 dpt) / (-1.0 dpt) = 1.5
(-1.5 dpt) / (-2.0 dpt) = 0.75

Note that the term diopter ratio is often used interchangeably for diopter gap, for example when talking about reducing a correction while keeping the gap the same. This can also be expressed as a percentage difference between the two diopter values (e.g. the 0.5 dpt difference between the right and left eyes here is equivalent to 0.5 dpt / |-1.5 dpt| = 0.33 or 33%).