In equation form, this is P=1f P = 1 f , where f is the focal length of the lens, which must be given in meters (and not cm or mm). The power of a lens P has the unit diopters (D), provided that the focal length is given in meters. That is, 1D=1m, or 1m−1 1 D = 1 m , or 1 m − 1 .