Charles Steinmetz (csteinmetz@redneck.efga.org)
Mon, 05 Apr 1999 07:11:42 +0000
Jewelia suggested that my observations about the differences between
point-ish (or collimated) light sources and diffused or planar sources are
"simplistic" and questioned whether investigations into such things are
important. She concluded by saying that light banks are the way to go, and
that adjusting other parameters can always get one what one wants. It is
precisely because the last assertion is not true that I have been at pains
to point out where light banks can let us down and what to do to mitigate
the problem.
First, I am not suggesting that there aren't a zillion things that affect
the total look of a print. Further, I agree that exact correlation between
the zillion causes and the total visual effect is much too complicated to
understand fully. That said, I think it is fruitful to isolate individual
print effects that are correlated with relatively few variables. Simple
(relativey), yes. Simplistic, no. Important if we wish to control our
processes so that substantial parts of the total visual effect can be
achieved by design rather than by accident.
The effect I'm talking about -- undercutting of the negative densities by
light arriving at all angles, which softens and blurs negative details on
the print -- is no revolutionary observation. It was well known to
glass-plate photographers last century, and is the primary reason for
double-transfer carbon printing (if not for the undercutting, they simply
would have flipped their negatives and printed through the glass to get the
image the right way 'round with one transfer). It is also the basic method
used to generate unsharp masks. Unfortunately, the fine details lost by
undercutting, like those lost by inaccurate focus, cannot be retrieved by
other means.
The question Keith originally posed was whether the differences between
point-ish and diffused light sources were visible in real alt prints made
with the two types of light sources. I answered that this particular
effect -- softness and blurring due to undercutting -- is routinely visible
to me, using negatives with lots of fine detail and printing on gelatin
POP, carbon, and Pt. I also reported that interposing a mylar barrier
layer between the negative and the paper, which some printers do to keep
the sensitizer off their negatives, exaggerates the effect. I opined that
with the mylar barrier nobody could miss the difference between prints made
with pointish and diffused sources, using a detailed negative and printing
on a medium capable of resolving fine detail.
Jewelia questions whether my observations really reflect this effect, or
whether they reflect other differences -- for example, the spectral
filtering of the mylar. First, adding mylar is not necessary -- the effect
can be clearly seen by flipping a negative over and printing through the
base, which introduces no additional filtration. (Try this with a glass
plate and a light bank. I used to show a couple of images made this way.
The effect is surreal with the right negative). Second, softening or
blurring of details looks different than contrast or color effects, and
what I see is soft or blurred details.
To summarize, the effect varies with the pointyness of the light and with
the separation between the negative emulsion and the printing material. It
gets worse with a less pointy light source and a greater separation, and
better with a more pointy light source and less separation. It appears
even when the spectral composition and intensity of the light do not change
(as shown by the flipped negative and by my experiment with opal glass in
the plate burner). Whatever other effects there are (due to spectral
differences between the sources, for example), THIS effect is discrete and
is correlated with the pointyness of the exposing light.
Now, the big question: Who should be concerned about it?
As I have said, I mostly print in-camera LF negatives of subjects with lots
of small details, and want prints that show all the detail on the negative.
So, I tend to use printing technologies that preserve fine detail: gelatin,
albumen, and collodion POP, carbon, and Pt coated on very smooth paper.
With my negatives and methods, prints made with no barrier layer and
exposed with a light bank can be distinguished, by the softening due to the
undercutting effect, from prints made with no barrier layer and exposed
with an arc. It's not a huge difference, but enough that I will always use
an arc if I have it available. With a barrier layer, I'm confident that
anybody would notice the difference (again, using my negatives and printing
methods).
Many people don't shoot subjects with lots of fine detail to begin with, or
don't consider the fine details necessary for their artistic vision. A
sizeable contingent feel that fine details positively detract from their
artistic statements, and many people deploy the alt processes in ways that
do not maximize their ability to hold fine detail (primarily, by coating on
paper or other materials with varying degrees of surface texture, and by
coating so that the image "sinks into" the substrate). These workers will
probably not care about the greater rendering of detail possible with
pointish light sources, and may even consider pointish sources
counterproductive.
Those who want fine detail on their prints (and who choose printing
technologies that are capable of reproducing it) are well advised to eschew
barrier layers if they use light banks, or to seek out pointish light
sources if they feel they must use a barrier layer.
Best regards,
Charles
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