"Acutance"?
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From: pure-silver-bounce@xxxxxxxxxxxxx
[mailto:pure-silver-bounce@xxxxxxxxxxxxx] On Behalf Of `Richard Knoppow
Sent: Monday, February 08, 2021 2:19 AM
To: pure-silver@xxxxxxxxxxxxx
Subject: [pure-silver] Re: More (Semi) Standing Around
The edge/border effect is caused by developer reaction
products affecting the degree of development where high and low
densities are next to each other. However, while that is caused
by developer reaction products they are not the same as produce
the differential hardening. Two separate effects. Both are
dependent on the reaction products and in turn by the type of
developing agent and the amount effect of the preservative in the
developer.
The nature of the emulsion will affect both. Since the
reaction products must travel through the emulsion by diffusion
the hardness and probably other qualities of the emulsion will
affect the result. The edge/border effect (I have forgotten a
word here) are caused by reaction products that diffuse laterally
in the emulsion in the vicinity of areas of high density. Some
developers generate reaction products that are accelerators of
development (hydroquinone is one) and some generate suppressors
such as bromide (metol). Its possible to formulate a developer
that has very little edge/border effect because the developing
agents tend to counteract each other. Pyro is not the only
developer that produces differential hardening, so does
hydroquinone, but in either case the effect can be exaggerated or
suppressed depending on the rest of the developer formula, its
dilution, the combination of developing agents, etc.
While (I will call it sharpness effect because I am losing
words right now) the sharpness effect may be desirable for some
conditions it is a real problem where the final image is the
result of several generations, as in motion picture photography.
Also, these effects are of fixed scale, that is, they are much
more visible on small negatives than large ones, with the
exception of course of the stain image.
On 2/7/2021 10:00 PM, Tim Daneliuk wrote:
On 2/7/21 11:43 PM, `Richard Knoppow wrote:
By "etched" do you mean a relief image, visible by light reflected from
the surface of the negative? If so, its caused by differential hardening of
the emulsion. Pyro is known for this and also hydroquinone to a lesser
extent. This is the same hardening effect used to make matrices for dye
transfer printing and for some other processes. Kodachrome was known for
having a relief image but I don't know what caused it.
The differential hardening of pyro and other developers can result in an
edge effect due to the variation in the index of diffraction of the emulsion
at the borders of light and dark areas.
The same conditions that cause a stain image tend to cause the relief
image although I think the reaction products responsible for them are not
the same ones.
Indeed, that's what I'm referring to. Looking back at my older APX 100
negs done in PMK, there is some such slight effect, but the effect of
Pyrocat-HD on FP4+ is much more noticeable.
It is my understanding that the edge effect is caused by the exhaustion
of very dilute developer. Are you saying this only happens with Pyro-based
developers or might we see the same thing with, say, Rodinal or very dilute
HC-110?
Interesting you should mention Kodachrome. I do recall it having a very
visible and present relief image. I have never much been one to shoot
color, but the K25 product was among the best color film I ever saw (some
Polaroid films being the other). I actually just watched the Ed Harris
film "Kodachrome" again the other day. A lovely movie that features Dwayne's
Photo's last run of processing of Kodachrome. A Leica M figures in a number
of scenes.
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