Re: gum curves
I'm beginning to gather that a person could get lost in the minutia
of this curve generation stuff and never get out again; now I'm being
told (elsewhere) that it's possible my scanner may be incapable of
accurately capturing the highlight end of the scale and for that
reason ChartThrob may not be giving me an accurate curve. I hope this
isn't true, because I paid rather a lot of money for it as scanners
go these days, but at any rate I think I've reached the point of
diminishing returns on this issue for the moment. Maybe when the
open source curve generator materializes, I'll give it another shot,
but in the meantime, I'd rather be printing gum.
Katharine
On Nov 12, 2006, at 10:12 AM, Katharine Thayer wrote:
On Oct 31, 2006, at 5:24 AM, Loris Medici wrote:
Thanks for that Sandy. All that you've wrote tells me that I'm in the
right direction. I've just started to adjust my gum / pigment /
dichromate / exposure amnts. aiming to get the same amnt. of steps
(or
close to each other) on step tablet test prints for each color I've
chosen - hoping this will equalize the contrast of each color layer.
Then I will start to calibrate a curve for each printing protocol.
Doing
so, I hope to get a fairly balanced result in the end...
Absolutely no
problems if I don't; it will be a good exercise anyway! :)
Yes, it will be a good exercise, and I'll be interested to see if
it gets you a good color balance in the end. It depends to some
extent on the yellow you choose, but I really doubt that any yellow
can be made to print with the same contrast and range as a cyan, no
matter how you manipulate other variables. A pigment can only do
what a pigment can do. And if you (borrowing Judy's image) tie two
hands behind your back by limiting the tonal scale of your blue and
red to match the yellow, then I don't see how you could get
anything but a pale, anemic tricolor print. But I stand ready, as
always, to be persuaded by evidence to the contrary.
As to whether the right curves (profile) can save an unbalanced
pigment set and make it produce true color, certainly that's
possible: the default CMYK profile in Photoshop is designed to
correct the lack of balance in process inks for commercial printing
(although in that case it also requires the black to bring
everything into balance). But to generate a profile like that
would require much more sophisticated techniques than we're talking
about here. The SWOP profile was developed for all the inks in
combination with each other; that's how they learned that a profile
that is skewed toward cyan is the right profile to give true color
with those particular inks. But in our case we're talking about
developing profiles for each of the colors individually and
expecting magically that this will result in true color balance
when the colors are printed together; I don't see how this could be
a realistic expectation. You can hold back color density with a
curve, but you can't add color density beyond what a given mix can
print from a clear negative, so the only way you can possibly
balance an unbalanced set is to balance to the lowest common
denominator, and I don't see how that could result in balanced color.
What I've learned from my short but intense study of curves is what
I've always known; the curves simply provide graphic
representation of well-established gum facts. For example, when
you get beyond a certain pigment concentration, no curve will
allow you to get good separation in the darks and highlights
simultaneously. Which is just another way of saying what we've
always known: you can get a good DMax with a heavy pigment
concentration of certain pigments, but such a pigment mix will
always give you a very contrasty print. As I always say, you can
have drama or subtlety, but not both in the same printing. This is
true with or without curves; the curves simply verify the fact.
With somewhat less pigment, you can get better separation at the
ends of the scale by steepening the ends of the curve, but to do
so you must necessarily flatten the midtones to accomplish this, so
you sacrifice some midtone separation, and you've already
sacrificed DMax with the dilution of the pigment. There is a
concentration at which you can get good separation throughout the
scale (with some pigments, not all) but again, you have to trade
DMax for separation. There is a point of paleness of pigment
where the midtones are already as flat as they can get; to steepen
the ends of the curve any further would reverse the midtones, so at
that point, the ends of the curve have to come in, in order to
produce a viable curve. So pale pigments, like most yellows, or
dilute concentrations of darker pigments, will have a fairly flat
curve where one or both of the endpoints don't go all the way to
the corner. But this is just another way of saying that pale
pigment mixes will print fairly flat and won't give us much DMax,
and we knew that without going to all the trouble of generating a
curve to show us that. In each case you can verify the truth of
the curve, and of the pigment mix, by applying the curve to the
calibration chart and printing that, as Kees helpfully
recommended. Where the ends of the curve have been steepened to
improve tonal separation at the ends of the scale, the midtones
will be flatter, both in the curve and in the tonal representations
on the calibration chart, and so forth. You never get something
without giving up something else. I've come to these conclusions
not only from my own observations, but from looking at the curves
and calibration charts that have been sent me by others that were
generated from other systems. Whether in the end what you
sacrifice is worth what you gain is a matter of personal choice and
preference; at this point I'm on the fence about it. But I will
say that after two weeks of printing these comparison images, the
flatter curved prints have come to look better to me than the more
contrasty uncurved prints, which wasn't true in the beginning.
So as not to lose the forest for the trees, I should probably say
again explicitly that the main thing I've learned from this is that
I still haven't seen a combination of pigment/concentration and
curve that would allow one to print a dark DMax and nicely spaced
tonal gradation throughout a long tonal scale, in one gum coat.
The comments above relate to monochrome printing only; I haven't
begun to try the individual curves in tricolor printing, to test
the hypothesis that one can automatically produce truer color with
curves than without.
Katharine
Regards,
Loris.
-----Original Message-----
From: Sandy King [mailto:sanking@clemson.edu]
Sent: 31 Ekim 2006 Salő 15:07
To: alt-photo-process-l@usask.ca
Subject: Re: gum curves
..
One of the advantages of making separations with digital negatives is
that you can actually make curves that will give true color from what
are essentially unbalanced tissue, pigment or dye sets. However, this
involves making curves for each color that correct or compensate for
the higher or lower contrast of the sets. I would not claim that this
can not be done without a sophisticated method of making curves, but
it would certainly be very hard to do so.
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