Industrial demands for integrated circuits of higher speed and complexity have required the development of advanced lithographic exposure tools capable of sub-half micron resolution over increasingly larger fields. To this end, i-line and deep-UV tools employing variable, high numerical aperture (NA) objectives are being aggressively developed. The design and manufacture of these advanced optical systems has also grown in complexity, since tighter tolerances on resolution and image placement must be maintained over the larger lens field. At the same time, usable focus and exposure latitude must be retained. The influence of lens aberrations on image formation under different illumination conditions, along with their non- intuitive nature has required the development of simulation tools that allow both the designer and the user of these systems to better understand their implications. These tools can be used to investigate and optimize the lithography process, including the effects of emerging technologies such as phase-shift masking, oblique illumination, and frequency plane filtering. This paper presents a method for determining the effects and interactions of various aberrations and illumination conditions using a statistically designed experiment. Fundamental differences in the way the aerial image is formed when varying the pupil energy distribution in the presence of aberrations are presented, as are examples of some of the more interesting effects.