The general theory of optically compensated varifocal systems is applied to a three-component system composed of two movable components placed on either side of a fixed component. The two movable components are displaced in unison to change the over-all focal length of the system. The three-component system is analyzed in detail and an iteration method for the solution of the varifocal equations is developed which enables us to determine the Gaussian parameters of the system for any predetermined focal range in a matter of a few minutes (without the use of computers). Using this method, explicit expressions are found for the approximate values of the parameters of the optimum three-component systems as functions of the focal range. A numerical example is given to illustrate the iteration method.
LEONARD BERGSTEIN and LLOYD MOTZ, "Three-Component Optically Compensated Varifocal System," J. Opt. Soc. Am. 52, 363-370 (1962)