1. A method for detecting defects on a wafer, comprising: generating output for a wafer by scanning the wafer using first and second different values for focus of an inspection system and the same values for all other optical parameters of the inspection system; generating first image data for the wafer using the output generated using the first value for focus and second image data for the wafer using the output generated using the second value for focus; combining the first image data and the second image data corresponding to substantially the same locations on the wafer thereby creating additional image data for the wafer; and detecting defects on the wafer using the additional image data.