ABSTRACT
Most photographic lenses use numerous elements of different refractive indices, curvatures, thicknesses and separations. The elements are arranged in groups and components. Alteration of separation between elements alters equivalent focal length, and is the principle of the zoom lens but is also useful in much simpler cases of illumination systems. The spacing of two elements is critical to the behaviour of the resultant system, whose properties may then bear no resemblance to those of the two individual components. In general, additional elements are needed to increase the number of times the moving focal plane coincides with the reference focal plane, and reduce the defocusing error. An alternative optical method uses two large aperture lenses in tandem with front elements facing each other. Both lenses are set to infinity focus. The image may have to be conducted some considerable distance by an optical relay system consisting of a train of spaced elements.
