1. A method of removing semi-transparent artifacts from digital images caused by contaminants in the optical path to the camera, comprising: a) accumulating a plurality of contaminated images to produce an average image that isolates spatially fixed, temporally persistent features in a background that converges to a local temporal low frequency measure of the images; b) downselecting semi-transparent artifacts from said isolated features; c) retouching the semi-transparent artifacts to resemble the background around the artifact to produce a retouched image; d) computing at least one correction power for each artifact pixel as a non-linear coefficient between the average image and the retouched image; and e) correcting each contaminated image by raising each artifact pixel to its correction power to produce a corrected image.