# Optical power

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Degree to which an optical system converges or diverges light

For magnifying power, see [Magnification](/source/Magnification). For the power of light, see [Radiant flux](/source/Radiant_flux) and [Luminous flux](/source/Luminous_flux).

Illustration of the relationship between optical power in [dioptres](/source/Dioptre) and [focal length](/source/Focal_length) in metres

In [optics](/source/Optics), **optical power** (also referred to as **dioptric power**, **refractive power**, **focal power**, **focusing power**, or **convergence power**) is the degree to which a [lens](/source/Lens_(optics)), [mirror](/source/Mirror), or other optical system converges or diverges light.[1] It is equal to the [reciprocal](/source/Multiplicative_inverse) of the [focal length](/source/Focal_length) of the device; high optical power corresponds to short focal length. The SI unit for optical power is the [inverse metre](/source/Inverse_metre) (m−1), which is also called a *[dioptre](/source/Dioptre)* (symbol: dpt or D) when used as a unit of optical power.

## Explanation

The optical power P of a device is related to its focal length f by *P* = 1/*f*.[2]

[Converging lenses](/source/Converging_lens) have positive optical power, while [diverging lenses](/source/Diverging_lens) have negative power. When a lens is immersed in a [refractive medium](/source/Medium_(optics)), its optical power and focal length change.

For two or more [thin lenses](/source/Thin_lens) close together, the optical power of the combined lenses is approximately equal to the sum of the optical powers of each lens: *P* = *P*1 + *P*2. Similarly, the optical power of a single lens is roughly equal to the sum of the powers of each surface. These approximations are commonly used in [optometry](/source/Optometry).

An [eye](/source/Eye) that has too much or too little refractive power to [focus](/source/Focus_(optics)) light onto the [retina](/source/Retina) has a [refractive error](/source/Refractive_error). A [myopic](/source/Myopic) eye has too much power so light is focused in front of the retina. This is noted as a minus power. Conversely, a [hyperopic](/source/Hyperopic) eye has too little power so when the eye is relaxed, light is focused behind the retina. An eye with a refractive power in one [meridian](/source/Meridian_(vision)) that is different from the refractive power of the other meridians has [astigmatism](/source/Astigmatism_(eye)). This is also known as a cylindrical power. [Anisometropia](/source/Anisometropia) is the condition in which one eye has a different refractive power than the other eye.

## See also

- [Accommodation of the eye](/source/Accommodation_of_the_eye)

- [Lens clock](/source/Lens_clock)

- [Lensmeter](/source/Lensmeter)

- [Plate scale](/source/Plate_scale)

- [Vergence](/source/Vergence_(optics))

- [Vertometer](/source/Vertometer)

## References

1. **[^](#cite_ref-1)** Basu, Dipak (2018-10-08). [*Dictionary of Pure and Applied Physics*](https://books.google.com/books?id=-QhAkBSk7IUC&pg=PA210). CRC Press. [ISBN](/source/ISBN_(identifier)) [978-1-4200-5022-6](https://en.wikipedia.org/wiki/Special:BookSources/978-1-4200-5022-6).

1. **[^](#cite_ref-2)** Greivenkamp, John E. (2004). *Field Guide to Geometrical Optics*. SPIE Field Guides. Vol. FG01. SPIE. p. 7. [ISBN](/source/ISBN_(identifier)) [0-8194-5294-7](https://en.wikipedia.org/wiki/Special:BookSources/0-8194-5294-7).

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