Power of a Lens System

The power of an axially symmetric lens or system of such lenses is a parameter that characterizes the refracting capacity of the lens or system. It varies inversely with the focal length of the system: ɸ = n’/f’ =—n/f, where n’ and n are the refractive indexes of the mediums located, respectively, in back of and in front of the system and f’ and f are the back and front focal lengths of the system as measured from the system’s principal planes. For a system in air (n = n’ ≈ 1), ɸ equals 1 /f’. It follows that the more strongly a system or an individual lens refracts light rays, or the smaller the focal length, the greater the power of the system. Measured in diopters (m^-1), the power is positive for converging systems and negative for diverging ones. The concept of the power of a lens or system of lenses is widely used in eye dioptrics and in the optics of eyeglasses.