Re: Many are Cold, Few are Frozen

["Dan Post" <dwpost@email.msn.com>]

The reason for cooling the film in astrophotography is to minimize what we
normally think of as "reciprocity failure"; we know that if we expose for
1/2 second at one light level, then theoretically, if you were in a light
situation where the illumination on the film was so much lower that you
would theoretically need, say 2 minutes, then there would be a problem with
molecules of the photosensitive material becoming "unexposed". When a photo
hits a molecule of the silver-halide, it displaces an electron- eventually,
this excited electron ( oooh, that sounds so sensuous!) falls back into un
unexited state... Hence the caution to have your film develpoped promptly!
Now, with astro photography, the photons are so few and far between, there
is a point where the net excited electrons in a given time period would
equal the number of electron returning to there unexcited state, and exposre
would essentially stop, you'd never get out of the "toe" of the exposure
curve. Consequently, the film holders, and I think too, in the case of some
CCDs used in astrophotography are chill to the point that the excited
electrons  once displaced will not randomly fall back to lower state of
energy. This "unexposure" is caused by random atomic movement ( Brownian
motion?) and that chilling to 4-5 degrees Kelvin sorta put a damper on that.
Under normal situation, so many excitations take place in the film, that it
would take a VERY long time for it to revert to anything resembling an
unexcited state, though we are cautioned to get the film developing promplty
because there are some deliteriuos effects from heat and other "radiation".
I have heard, however, that film  with the "T-grain" technology does not
benefit at all from freezing. I this from a Kodak rep years ago.
From the standpoint of someone working in photofinishing, the reason there
are "Pro" color films, as opposed to "Amateur" films is that the latitude
for most color films of the "Pro" type is much less than that of others-
most amateur film is manufactured so that changes to the sensitivity of the
three sensitive layers will change evenly enough that the "slopes" of those
layers stay relatively close, all conditions remaining within reason. PRO
film has a narrower range of tolerance, and cooling keeps the changes
slowed. Also- and this is a sore point, some folks insist on using what they
think is professional film, Like Kodak PJA, and wonder why their prints
sometimes are off! First, it's a film designed for scanning, and the
response slopes are not even meant for printing!
So, IMHO, semi-professional, use the film that gives you the colors you
want. Keep it cool, or at least out of hot cars, and where there is an
abundance of hydrocarbon vapor. If you buy it by the brick, like I do, I
just keep it in the little frige I keep my paper in, at under 55 degrees
F... No need to freeze unless you're keeping it for years and years!
Oops- I just fell off m' soapbox!
- -----Original Message-----
From: Jacques Bilinski <jbilin@axionet.com>
To: leica-users@mejac.palo-alto.ca.us <leica-users@mejac.palo-alto.ca.us>
Date: Sunday, 16 November, 1997 1:42 AM
Subject: Re: Many are Cold, Few are Frozen

>
>> >As far as I'm concerned 3200 even in a freezer and frozen solid will be
>> >affected. Is this technically true? I haven't got a clue. I just know it
>> >can be in the freezer for 6 months and when you go to use it and compare
>> >negs after being in the freezer to negs that were exposed when you first
>> >got the film and there is a distinct overall "fog" look.
>>
>> ANY film will be fogged by cosmic rays over a period of time.
>
>In astronomy they sometimes use special cameras in which the emulsion is
>chilled with liquid nitrogen. I was under the impression that film became
>more sensitive (presumably to both visible light and cosmic rays) at very
>low temperatures. Do any of the amateur astronomers on the LUG know
>anything about this?
>