From: Bob Kiss (bobkiss@caribsurf.com)
Date: 06/18/01-05:15:45 AM Z
DEAR LIAM,
I feel the problem may be more complex.
For Silver Gelatin materials High Intensity RLF usually compresses the
high exposure areas while leaving the middle and lower exposure areas (in
any given scene) at "normal" contrast with an net decrease in contrast
necessitating an increase in both exposure AND development. Low Intensity
RLF usually does the opposite, i.e., it reduces the effectiveness of the
lower exposures while leaving the middle and higher exposure areas (in any
given scene) at "normal" contrast resulting in an effective INCREASE in
contrast necessitating an increase in exposure but a decrease in
development. All of this results from the time and intensity dependent
conflicting processes of latent image formation, interstitial silver ion
migration, of electrons to the interior of the AgX crystal and
rehalogination.
Hence, speaking strictly of RLF (not factoring in Auto-masking) higher
source intensities should result in reduced contrast and lower source
intensities should result in higher contrast *if they are outside the "flat"
region where reciprocity doesn't fail*.
This seems contrary to what people are experiencing...yes? Even the
instruction sheets with Chicago Albumen Works POP state that contrast can be
increased by leaving the negative/POP sandwich in the printing frame under
normal room fluorescent lights for very long exposures. I haven't tried it
so I cannot say.
So I am afraid the problem is more complex...perhaps spectrum as Jeffrey
suggests, or some other causes. And, of course, this may be totally
academic as Cyanotype is NOT a silver/gelatin process.
CHEERS!
BOB KISS
----- Original Message -----
From: Liam Lawless <lawless@ic24.net>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Saturday, June 16, 2001 9:21 PM
Subject: RE: contrast control, the sun, and cyanotypy
> As we all know, silver emulsions are most sensitive to light intensities
in
> a certain range, within which exposure = time X intensity. Outside this
> range (i.e. for very low or high intensities), the relationship no longer
> holds (reciprocity failure): the photographic effect of a given amount of
> exposure is less than it would be when reciprocity obtains, and exposures
> need to be increased acording to the emulsion type and how far the light
> intensity falls outside the normal range.
>
> Seems pretty likely, therefore, that sensitisers for alt processes suffer
> similarly. If a source such as the sun is bright enough to cause
reiprocity
> failure, this could explain the loss of contrast. Remember, too, that the
> light intensity will be highest under the clearest areas of the negative
> (the areas which print darkest), and that this is where the effects of
> reciprocity failure would be greatest.
>
> Just a guess.
>
>
>
> Liam
>
> -----Original Message-----
> From: shannon stoney [mailto:sstoney@tnaccess.com]
> Sent: 15 June 2001 03:47
> To: alt-photo-process-l@usask.ca
> Subject: contrast control, the sun, and cyanotypy
>
>
> Hi. Another cyanotype question, which may apply to other processes as
> well. I have been using the sun to expose my cyanotypes. When there are
no
> clouds, the exposure takes about three minutes, and I get about eight
steps
> on the step tablet. But, if it's cloudy, the exposure may take seven to
> eight minutes, and I get more steps! Has anyone else observed this
> phenomenon?
>
> --shannon
>
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