From: Richard Knoppow (dickburk@ix.netcom.com)
Date: 08/05/00-08:57:03 PM Z
At 09:31 PM 08/05/2000 -0400, you wrote:
>The brief discussion about the sun being solarized in an Ansel Adams
>negative reminds me of various pinhole negatives by students and myself,
>both film and paper, in which the sun is solarized. This leads me to a
>question I've had for a long time. Does the characteristic curve repeat
>itself with large doses of light? Could the sun change back and forth from
>being solarized to being opaque on the negative to being solarized again? I
>have noticed pieces of fogged film lying around at school that are different
>shades of pink to tan depending on which parts were sticking out from under
>others and thus exposed to even more light. This film does not always give a
>uniform black when developed for chiché-verre purposes. I imagine the
>characteristic curve of this film as a wave form repeating with diminishing
>amplitude and decreasing sensitivity but nevertheless repeating. Working
>with duplicating film has caused me to think about the far side of the curve
>since the film is preexposed to the shoulder and the image is formed going
>downhill. I hope someone understands my question and can answer it.
>
>Thanks,
>Sarah
>
This is exactly what happens. The characteristic curve increases to a
maximum and stays there for an interval, then reverses. If enough
overexposure is give you get a partially reversed image. Modern emulsions
are formulated to eliminate this effect as much as possible, its likely
some films never reverse. The physical cause is a recombination of charges
when there is enough overexposure.
Another effect frequently seen is black lightening. This is partially
from the above effect and another where flashing causes subsequent reversal
(forgotten the name at the moment, Claydon effect?).
---- Richard Knoppow Los Angeles,Ca. dickburk@ix.netcom.com
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