> What exactly does the unsharp mask do to "restore original appearance"?
> Doesn't it make the picture, eh well... unsharp?
>
> Henrik Dahl
No idea what 'Unsharp Mask' does in Photoshop, but I'm pretty sure it's
not what it does in the darkroom. It's a technique used to fantastic
effect by David Malin, to extract more information from astronomical
negatives. He makes a blurred positive of his original monochrome
negative, then copies the original through the unsharp mask, which gives
a print showing much more of the dynamic range of the negative than is
normally possible. See these pages for before and after examples:
http://www.ast.cam.ac.uk/AAO/images/captions/aat019b.html (before)
http://www.ast.cam.ac.uk/AAO/images/captions/aat019a.html (after)
And this page for technical info:
http://www.ast.cam.ac.uk/AAO/images/general/technical.html
Nothing to do with actually sharpening the image, and I've no idea why
photoshop calls a sharpening filter a misleading name which refers to an
entirely different effect!
Anyway, that was a bit of an aside, really. Image sharpness is lost
when you scan because you are undersampling the image - ie putting
several bits of image information into each pixel. The information is
merged and a loss of sharpness results. All these sharpening filters
and whatnot don't really restore anything, they just make it look
better, purely cosmetically.
Roger
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