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Originally posted by davidbiedny
The "Unsharp MasK" filter in Photoshop is based on a digital simulation of an analog process that originated in Europe in the mid-thirties, and did indeed involve a blurred interpos of a piece of film.
I have a somewhat decent understanding of the various processes involved in photographic and digital sharpening, and I also have some knowledge of the limitations of desktop-level tools, especially free ones.