The general theory of optically compensated varifocal systems is applied to the case of a four-component system consisting of four alternate stationary and movable components. The second and fourth components, counting from the object side, are interconnected and displaced in unison to change the over-all focal length of the system. An iteration method for the solution of the varifocal equations is developed which enables the determination of the Gaussian parameters of the system for any predetermined focal range in a matter of minutes (without the use of computers). Using this method, explicit expressions are found for the approximate values of the parameters of the optimum four-component systems as functions of the focal range. A numerical example is given to illustrate the iteration method.
LEONARD BERGSTEIN and LLOYD MOTZ, "Four-Component Optically Compensated Varifocal System," J. Opt. Soc. Am. 52, 376-383 (1962)