Though I've read Wayne Fulton's excellent scanning tips document (Wayne:
thanks for plain English and such a clear and thoughtful service) I am
still a tad confused about the difference between using an image
sharpener and using the USM option. Can anyone lay out just a few very
clear tips and comparisons to help? Maybe its the terminology I'm stuck
on. Why would an 'Unsharp' tool do a better job of sharpening than
something called a 'Sharpening' one .
No, it is not an "unsharp" tool. USM is just a name, unfortunate name
perhaps, since it is instead a method of sharpening.
Yes, it is primarily terminology, although it does describe a method. The
USM filter is a sharpening tool, and nothing else than a sharpening tool,
a very good one for photos, perhaps the best one for photos.
The name Unsharp Mask comes from non-digital older days when it was
implemented in the darkroom by first making a copy on negative film by
exposing that film copy with an underexposed and slightly out of focus
reversed image, which was then used as an aligned mask to sandwich and
overlay the original negative film during final print making. This blurred
the sharp edges slightly in a controlled way, however the apparent final
result effect was the opposite, primarily enhancing the contrast at the
edges in the final print, making it appear more sharp, more defined.
The actual idea was this: Underexposed means the copy mask was closer
to being clear film than dark film, clear having little overall effect
generally. The Reversed mask (the negative film copy is now positive,
but is sandwiched with the original negative) means the mask is darker in
the light areas and lighter in the dark areas, the opposite of the natural
situation. This sandwich will be inverted with the original negative when
it is printed. Blurred means this mask has slight but subtle differences
in the original so that there is a result, but primarily only at the edges
of contrast difference. It is blurred (meaning slightly wider edges, so
the finest detail may be reduced) but nevertheless, the result is still
sharper in appearance, and we all definitely like the sharpening very much
(so long as we dont over do it).
In the final print, this sandwich widened the sharp edges slightly (the
blur), however it did it in the special way that the dark side of the
result edge was even darker, and the light side of the result edge was
even lighter. This has the opposite appearance from blurring. The extent
is only that blur width, but it is a greater width than the original edge
width. This makes the edge more visible, wider but more contrast, and
with the apparent effect of it appearing much more sharp than it
previously appeared. This greater edge visibility is sharpening. The
slighter greater edge width is necessary to add this sharpening effect (so
dont over do it, it is easy to over do it).
Today this sharpening effect is much easier done with digital images. But
in the digital image, the dark side of the edge is still even darker, and
the light side is still even lighter, working in the same way as the early
days USM technique.
Short answer, USM is definitely sharpening, an excellent tool for photos,
but it is probably best to think of it as just a name for the best such
tool for photos.
