1. A method for inspecting a photo-mask, comprising: receiving an inspection image corresponding to the photo-mask, wherein the inspection image is determined using an optical imaging system, and wherein the inspection image includes less spatial information than is present in the photo-mask; and determining an estimated pattern corresponding to the photo-mask at an object plane of the optical imaging system, wherein the determining involves restoring image information that was lost when the inspection image was determined, wherein the determining uses a transfer function of the optical imaging system, and wherein the estimated pattern is determined using an inverse optical calculation in which the inspection image is in an image plane of the optical imaging system and the estimated pattern is at the object plane of the optical imaging system.