# Difference between revisions of Diopters

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

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+ | |||

+ | ==Gap and ratio== | ||

+ | |||

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

+ | |||

+ | * ''diopter gap'': 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 | ||

+ | |||

==References== | ==References== | ||

{{reflist}} | {{reflist}} | ||

[[Category:Articles]] | [[Category:Articles]] |

## Revision as of 16:59, 30 July 2020

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}.

- 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 .

- 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.

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*: 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