Wynne-Dyson optical system with variable magnification

A 1× Wynne-Dyson optical system for microlithography having a variable magnification is disclosed. The 1× Wynne-Dyson optical system has first and second prisms, and a positive lens group that includes a split lens having first and second split lens elements that reside adjacent the first and second prisms, respectively. The first and second split lens elements are axially movable to change the magnification by up to about 500 parts per million. An adjustable positive lens group for a 1× Wynne-Dyson optical system is also disclosed, wherein the positive lens group allows for small changes in the optical system magnification.