Optical power (dioptric power or refractive power) is the degree to which a lens or mirror converges or diverges light. It is equal to the reciprocal of the focal length of the device. The dioptre is the most common unit of measurement of optical power. The SI unit for optical power is the inverse metre (m-1). For two or more thin lenses in contact, the optical power of the combined lenses is very well approximated by the sum of the individual optical powers of each lens. Optical powers are commonly used to characterize lenses used in ophthalmology and for geometric ray tracing. An eye that has too much or too little refractive power to focus light onto the retina has a refractive error. A myopic eye has too much power so light is focused in front of the retina (e.g., the focal length of the lens is too short). Conversely, a hyperopic eye has too little power so when the eye is relaxed, light is focused behind the retina (e.g., the lens's focal length is too long). An eye with a refractive power in one meridian that is different from the refractive power of the other meridians has astigmatism. Anisometropia is the condition in which one eye has a different refractive power than the other eye.
