ABSTRACT
An understanding of residual aberrations is useful in describing the types, merits and imaging limitations of camera lenses. An 'ideal' lens forms geometrically accurate images but actual lenses do not since the refractive index of glass varies with wavelength, lens surfaces are usually spherical in shape and because of the wave nature of light. For transparent media, shorter wavelengths are refracted more than longer wavelengths, causing spectral dispersion and dispersive power. The variation of focus with wavelength for an uncorrected lens is an approximately straight line; that of an achromatic lens is approximately a parabola. Plastics are used in photographic lenses either as individual elements or as hybrid glassplastic aspheric combinations. The use of reflecting surfaces that do not disperse light, in the form of 'mirror lenses', offers another solution, but most mirror designs are for long-focus lenses only. The performance of a single-element lens is limited by the available number of optical variables or degrees of freedom.
