This paper explores fundamental imaging phenomena that become appreciable as the numerical aperture (NA) of imaging lenses increase beyond 0.5. A high NA imaging model is formulated based on plane wave decomposition of the imaging lens exit pupil. The model shows that for NAs above about 0.55, linear polarized illumination gives rise to "astigmatic" effects in the image. Photoresist exposures in polarized light on a 0.55 NA stepper show ≈1% x-y linewidth differences in experimental tests. © 1992.