The optical power of a device is related to its focal length by .
Converging lenses have positive optical power, while
diverging lenses have negative power. When a lens is immersed in a
refractive medium, its optical power and focal length change. For two or more
thin lenses close together, the optical power of the combined lenses is approximately equal to the sum of the optical powers of each lens: . 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. 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. This is noted as a minus power. Conversely, a
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 that is different from the refractive power of the other meridians has
astigmatism. 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. ==See also==