The two main problems facing the designer of varifocal lens systems are compensation for image shift and correction of aberrations for each zoom position. Most designers have chosen the easy and obvious way of compensating for image shift by using mechanical nonlinear cams, gears, and linkages, and have concentrated their efforts on aberrational correction only. The alternative is shift compensation by optical means.
F. G. BACK and HERBERT LOWEN, "The Basic Theory of Varifocal Lenses with Linear Movement and Optical Compensation," J. Opt. Soc. Am. 44, 684-690 (1954)