High-quality imaging is essential for having the largest available process window for the various lithographic mask features, particularly those at the resolution limit of the lens. While lithographic lenses are designed to be 'diffraction limited,' shortcomings in the manufacturing process can introduce aberrations which affect imaging. The effects of such aberrations are explored via computer simulation, using exposure-defocus (E-D) diagrams and plots of linewidth versus focus for isolated and grouped lines. Data from a variety of lithographic lenses and the criteria for measuring lens performance are discussed.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.