A variable anamorphic lens system for a 35-mm SLR camera is described. Using a pair of achromatic prisms an anamorphic range of 0.75:1.33 is achieved. The lens in its unanamorphic state has a focal length of 100 mm and an aperture of f/5.6. Focusing from infinity to 0.1 magnification is made possible by separating out the positive and negative components of the nominally zero power front group.
Ian Powell, "Variable anamorphic lens for a 35-mm SLR," Appl. Opt. 22, 3249-3257 (1983)