The general theory of optically compensated varifocal systems is applied to the case of a two-component system consisting of a single movable component placed behind a fixed component. This is the simplest case of a varifocal system and one that can be treated in all its details by elementary analytical methods. It is shown that for a given focal range four different systems are possible in all of which the focal length of the movable component has the same magnitude. Of these four systems only two are of practical interest. It is demonstrated that if the optimum conditions are realized the image deviation can be reduced to a sufficiently small value so that the two-component varifocal system can be of practical value in some specific applications, as for example in projection systems and viewfinders, as well as in telescopes of small magnification range.
LEONARD BERGSTEIN and LLOYD MOTZ, "Two-Component Optically Compensated Varifocal System," J. Opt. Soc. Am. 52, 353-353 (1962)