Annular field projection optical systems have been used in production for the manufacture of circuits with feature sizes about 75% of the size of those produced by stepper lenses with the same diffraction limits. One factor contributing to this is the uniformity of imagery over the field inherent in these systems, both in and out of focus. The limits of optical microlithography will be determined by the smallest feature sizes that can be recorded with adequate focal range. For partially coherent illumination, the intensity/defocus (l-D) diagram of an optical system for a particular pattern can be used to give both the depth of focus for a given recording window and the recordability of a pattern as measured by the slope of the intensity distribution of the aerial image at the recording point. The inherent focal range advantage of annular field optical systems, when combined with their other advantages, makes them prime candidates for extending the range of optical lithography to feature sizes below 0.5 µm.