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, and wherein the determining uses a transfer function of the optical imaging system.