The lens strength in diopters is defined as the inverse of the focal length in meters. For a thick lens made from spherical surfaces, the focal distance will differ for different rays, and this change is called spherical aberration . The focal length for different wavelengths will also differ slightly, and this is called chromatic aberration . The principal focal length of a lens is determined by the index of refraction of the glass , the radii of curvature of the surfaces, and the medium in which the lens resides. It can be calculated from the lens-maker's formula for thin lenses.
Optometrists usually prescribe corrective lenses in terms of the lens power in diopters.