# 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}.

- 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*(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*^{[1]}, 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^{[2]} (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%).

## References

- ↑ The EndMyopia Blog, https://endmyopia.org/the-diopter-ratio-trap-dont-favor-one-eye/
- ↑ The EndMyopia Blog, https://endmyopia.org/reducing-diopter-ratio-diy-patching-solution-pro-topic/