Inoptics,optical power (also referred to asdioptric power,refractive power,focal power,focusing power, orconvergence power) is the degree to which alens,mirror, or other optical system converges or diverges light.[1] It is equal to thereciprocal of thefocal length of the device:P = 1/f.[2] High optical power corresponds to short focal length. The SI unit for optical power is theinverse metre (m−1), which, in this case, is commonly called thedioptre (symbol: dpt or D).
Converging lenses have positive optical power, whilediverging lenses have negative power. When a lens is immersed in arefractive medium, its optical power and focal length change.
For two or morethin lenses close together, the optical power of the combined lenses is approximately equal to the sum of the optical powers of each lens:P =P1 +P2. 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 inoptometry.
Aneye that has too much or too little refractive power tofocus light onto theretina has arefractive error. Amyopic eye has too much power so light is focused in front of the retina. This is noted as a minus power. Conversely, ahyperopic eye has too little power so when the eye is relaxed, light is focused behind the retina. An eye with a refractive power in onemeridian that is different from the refractive power of the other meridians hasastigmatism. This is also known as a cylindrical power.Anisometropia is the condition in which one eye has a different refractive power than the other eye.
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