From: Liam Lawless (lawless@ic24.net)
Date: 06/16/01-06:21:41 PM Z
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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