Re: Gum tonal range (was Miracle size for gum)
I've just started looking at responses to my post, and while I
appreciated Etienne's cogent response, the next response I glanced
at seemed to be responding to something I didn't say at all, so maybe
a general clarification would be helpful:
I was not arguing against the use of step wedges, for heavens' sake;
I would have thought that was quite clear when I said that the number
of steps printed is the useful measure for gum printing (besides,
after all, I use and post step prints all the time), but the number
of steps printed shouldn't be used to extrapolate any general rule
about print density. Step wedges are very useful, especially for
quantifying one's own practice. I was simply arguing against the
translation of number of steps, or the number of "stops" to a print
density range by multiplying by .15 or .30. Hope that point is now
perfectly clear.
Katharine
On Oct 11, 2009, at 11:01 AM, Katharine Thayer wrote:
Goodness, this thread has been confusing. It seems a perfect
example of what Judy was talking about the other day; I can't find
it now, but as I recall it was the post that made Paul snort a
peanut butter sandwich out of his nose. The main idea, written in
Judy's inimitably witty prose, was that people who come to gum from
platinum (I would broaden that to include silver as well) come with
a set of preconceptions that don't translate well to gum. I
couldn't agree more, and this thread is a good illustration of
what happens when you try to import conventions that work for
metal processes into a discussion about gum, where they don't apply
at all.
With metal processes there is a well-understood relationship,
which can be expressed in an equation that every photographer
knows by heart, between exposure, measurable amount of deposited
metal reaction product, and measurable amount of light reflection
from the print. One follows in a lawful way from the others, and
back again (in other words, besides being able to predict the print
log density of an area from the exposure, you can also start from
the reflection log density and translate it back into exposure).
In those systems, it makes perfect sense to say that if six
steps are printed from a Stouffer 21-step wedge, that will
translatein a lawful way to a tonal density range of log .9, but
in gum there is no such relationship between exposure, steps
printed, and tonal range, and such a statement is misleading, if
not meaningless. The attributed statement "Gum prints two stops"
is simply nonsense; it can't be connected to anything meaningful in
terms of gum. And then on top of that, there's also a confusion
in this thread between negative DR and print tonal range, which has
added to the confusion.
Loris has it right. With gum, the print tonal range is more a
function of pigment and pigment concentration than of negatives or
exposure. The maximum and minimum absolute densities are
determined by the pigment and the concentration of that pigment
in the emulsion. I have an example on my site of the same pigment
(PBk11 --iron oxide black) printed at six different pigment
concentrations, with a table showing the measured densities of the
lightest and darkest values in the print. The lightest pigment
load printed from log .10 to log .60, or a tonal range of .50.
The heaviest pigment load, which was deliberately overexposed and
then treated with violent methods of forced development to bring
out the lighter tones, had a DMax of log 1.00 and lightest tones
of .25, or a tonal range of 1.00; however the extreme methods of
forced development didn't produce a print with natural tonal
gradations, so I don't think it really counts; like a lot of early
gum prints it's just mostly dark with some light values forced out
of the darkness by power-washing and vigorous scrubbing.
The three pigment concentrations that fall into the range most
people would use for most gum applications (the lighter two of
these you'd use for one of two or three layers, say, and the
darkest for a one-coat gum print) the DMax ranges from .95 to 1.05
and the Dmin from .20 to .30, but in each case the tonal range
printed (difference between absolute density for darkest and
lightest values) was log .75.
This log .75 is a range of measured reflection density, but that
measure of optical density has nothing, I repeat NOTHING, to do
with stops of exposure. I'm repeating myself, but it bears
repeating; the equation that relates exposure to density of
reaction product to optical density of tonal values *does not hold
for gum,* and it makes no sense whatever to talk about tonal range
in gum, or even steps printed in gum, in terms of stops of exposure.
Yes, the Stouffer 21-step has incremental steps of log .15, or
"half a stop" for each step. This means that when the 21-step was
printed on the film, for every half stop of increased exposure the
density of the silver reaction product deposited on the film
increased by log .15 and the opacity of the film also increased by
that much. But that relationship, that holds for how the silver
was deposited on the film strip, does *not* hold for the printing
of the gum through the test strip, and does *not* mean that for
every step of gum printed, there is .15 more optical density in the
print.
I did that demonstration only with the one pigment, and I gave
back the borrowed densitometer, so I can't check to see if that .75
holds for other pigments; I suspect not, since there are few
pigments (lamp black is really the only other one) that can print
such a dark value) so it wouldn't be reasonable to state as a
principle that "Gum prints a tonal range of log .75" (although it
would be somewhat more reasonable than saying "Gum prints two
stops" which makes no sense at all). The fact is, and my point
here is, that it's really meaningless to try to make use of those
photographic conventions when talking about gum, on the contrary
it's misleading and confusing. To try to establish how many
"stops" gum will print is a question that has no answer, a
question that makes no sense.
To say many *steps* gum will print, without making the mistake of
translating the steps into stops of exposure or into density, is
more useful in a practical sense, but as experienced gum printers
know, this is influenced by a great many variables and will vary
widely across emulsions. So to state as a general rule how many
steps gum will print is also something of a chimera; the answer, as
for almost everything in gum, is "it depends."
Katharine
On Oct 10, 2009, at 1:02 PM, phritz phantom wrote:
hi chris,
i did tests for tonal range about a year ago and again two months
ago. i printed step wedges with saturated and 5% ammonium
dichromate. since i don't have a proper step wedge (yet), i can't
say how many steps, but for me i get about twice the tonal range
with saturated than with 5%.
with the (uncurved) chart throb scales and 5% am-di there appr,
2.5 lines between max. densitiy and white. while with the
saturated solution there are 5 or 6 lines between white and black.
another thing i noticed is that the 5% solution does significantly
clear better than the saturated (on the cheap paper i use). i
wouldn't really call it stain, but the whites of the 5% sheet are
noticably more brilliant. also i think the tonal range of the 5%
does break off more abrupt in the highlights. maybe it's just the
fine highlights that wash off quicker in the development, because
of the lesser light sensitivity of the 5% solution.
phritz
Christina Z. Anderson schrieb:
Loris,
Someone said in the literature recently that gum prints 2 stops
(log .6). I was surprised at how low this was, as I always hedge
my bets and say 4-6 stops, with 6 a stretch. Partly if you
don't clear with pot metabi you can get a false read of maximum
black because the brown stain of the dichromate can read a darker
step when it is not really hardened anymore "goo" on top. So I
always figured that the 2 stop person cleared and found that to
be true. Or maybe was guessing it from the seat of his/her pants.
But it's all kind of a moot point I suppose once you fit your
negative curve to the exposure/dichromate you use.
I use a 15% solution of am di (2 tsp to 100ml quick n' easy
approximate).
So Marek is figuring a 3 stop range, you a 4-5 stop range....
You say that with weaker di and greater exposure you are getting
a longer tonal range--do you have a picture on the web of that
comparison somewhere?
Chris
__________________
Christina Z. Anderson
http://christinaZanderson.com/
__________________
|