Abstract. A new approach for characterizing resolution and depth of focus (DOF) in optical microlithography is introduced. By examination of the interaction of the aerial image with the photoresist process, a metric of image quality is defined. The variation of this metric with feature size and defocus can be used to measure the resolution and DOF. The effects of various imaging parameters on DOF can then be determined. To further study focus effects in submicrometer imaging, the lithography simulation program PROLITH (the positive resist optical lithography model) is modified to account for defocus within the photoresist film.