Kenneth Sloan wrote
>>The technique is called "unsharp masking". Start with a slightly
out-of-focus negative. Make an even MORE out-of-focus POSITIVE
transparency. Now...print the sandwich and what do you get? A sharper
print. Why? Because the print is a blend of the original image (which
contains high and low frequency energy) and the unsharp mask (which
contains very little high frequency energy, but lots of low frequency
energy). Reversing the sense of the two image means that you have:
Image - UnsharpMask = "sharper" image<<
I've found in Michael Langsford's "Advanced Photography" this explanation:
<< Slight diffusion of the image means that over areas of finest detail a
simple "average" even tone is recorded on the mask, whereas borders of
larger tone areas record as tone changes. When unsharp mask and negative are
combined contrast is reduced in all areas except the very fine (almost
microscopic) detail, which more-or-less retains its original contrast. We
know how visual sharpness or definition depends as much on detail contrast
as actual resolution. A print from the combination, with its detail contrast
relatively greater than its general contrast, appears slightly sharper than
a print from the original negative. We have the curious fact that un unsharp
addition to a negative improves it's definition >>
I suppose that our "darkroom specialists" will have some point of view about
that, and probably some practical experience...
regards
marc
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