shadow detail loss on press, but not on proof

Dan Margulis <76270.1033@...>

Chris writes,

A close-up shot of a black person on a proof shows up with really good
detail. On press, there is no detail in the skin - it's gone. I haven't
seen the separations yet, but they describe what sounds
like a lot of detail on the black plate. To me it sounds like two things,
let me know if you think that sounds about right:>>

Given the numbers you describe, they have three problems: they are
underestimating dot gain at least in black, but probably overall as well;
they have too high of a black ink limit; and they don't realize that
Photoshop's built-in monitor preview overstates the power of black ink.

This is a CMYK-specific problem, an effect that isn't well known.
Regardless of sep settings, at some level of darkness the CMY channels max
out, and all the detail goes into the black. The CMY will look blurry or
even ghosted out. This point comes earlier as the total ink limit goes down
and also as the black ink limit goes up.

The numbers you quoted indicated that once they get to 60c50m50y that's
about it for the CMY. A shadow of
62/51/49/65 like you quoted is asking for a problem. It should be something
more like 75c65m65y50k. That way the detail's in all four channels, the
impact of misstating black dot gain isn't as severe, and the monitor won't
seem to lie so much.

A side question. If press dot gain were controlled and accounted for
during separation really well, wouldn't it be OK to use a medium black
generation as long as the black ink limit were suitable?>>

On a nonimpact printer, yes, on a press, no. On press, black is 1) the ink
with the highest dot gain; 2) the one where the density is most likely to
be something unexpected, because the pressmen are prone to judge the black
by the appearance of the type and not the pictures, and because the
presence of large black areas on the form (such as headline type) can alter
densities by themselves; 3) the one that has as much weight as the other
three put together, so that any error appears grossly magnified.

All this are true no matter what the black generation, but if you use light
generation the picture may well look *better* if the black density is too
high. Shadow detail will be lost but otherwise the picture will seem to
gain contrast. OTOH, if you use medium generation, there will be a modicum
of black ink in the fleshtones and other semisaturated colors, and these
will get muddy rapidly.

The cases where a heavier GCR is appropriate are usually those where the
subject is distinctly neutral or where for some reason you don't like the
idea of bright colors.

Or is it better with dark images with lots of shadow detail that needs to
be retained to keep black generation lower?>>

In images with critical shadow detail one normally has to exaggerate the
tail end of the black a bit. CMYK is too low-contrast to do nothing, and
the CMY channels have no detail in the shadows. Therefore, it's a major
hindrance to have more non-shadow detail in the black than necessary,
because we're apt to wipe it out in fixing the shadows. So, yes, you want a
lighter black generation in this case.

Dan Margulis