The power of a lens is given as the reciprocal of its:
Focal length (in metres)
The power of a lens measures its ability to converge or diverge light and is defined as \(P = \dfrac{1}{f}\), where f is the focal length expressed in metres.
Because the focal length must be in metres, the unit of power is the dioptre (D), where 1 D equals 1 m-1. A shorter focal length gives a higher power, meaning the lens bends light more strongly.
Thickness is not used in this relation; a lens can be thin yet have high power if its focal length is short.
Diameter (aperture) affects how much light a lens gathers, not its power, which depends only on focal length.
Radius (of curvature) determines the focal length through the lens-maker's formula, but power itself is the reciprocal of the focal length, not of the radius.
Hence, the power of a lens is the reciprocal of its focal length in metres.
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