One of the many things astrophotography teaches you is understanding of
reciprocity law failure in films. :-) You summerization ofr what
recirpocity failure is essentially right. This characteristic varies
greatly from one emulsion to another. Various methods of treating the film
also change this characteristic. One method, called hyper-sensitization,
drives out impurities and moisture by "cooking" the film at high temperature
is a hydrogen/nitrogen gas (forming gas) mixture. Another method is to
place the film under a high vaccuum and store with dessicant to keep all
moisture out of the emulsion. The camera body is modified to provide a dry
nitrogen gas purge to make sure the humidity in the air can't reach the film
during exposure. These somewhat extreme measures increase the film's
sensivity to (very dim) light, reduce the effects of reciprocity law failure
and often shift the color balance - sometimes favorably sometimes not ....
At the other extreme, very short exposures in modern films do not exhibit
such a problem. With the exception of Kodak's VPL film, an emulsion
specifically tuned for long (> 1 sec) exposures, there are no ill-effects
for exposure time up to 1/10,000 sec. Pretty quick :).
John P
______________________________________
My Grandfather taught me to live by two rules. Rule #1: Don't tell folks
everything you know.
-----Original Message-----
From: Pauls0627@xxxxxxx <Pauls0627@xxxxxxx>
To: olympus@xxxxxxxxxxxxxxx <olympus@xxxxxxxxxxxxxxx>
Date: Friday, September 04, 1998 6:03 AM
Subject: Re: [OM] long auto exposures
>In a message dated 9/4/98 2:31:56 AM Eastern Daylight Time,
>red_rover@xxxxxxxxxxxxx writes:
>
><< I've read of reciprocity failure, where film, when taken at long
> exposures, usually requires much more exposure then a meter calls for.
I've
> learned that my Om2s can take auto exposed pictures at lengths of up to 2
> minutes, but wouldn't this be an incorrect exposure due to reciprocity?
> Should I see how long the camera takes the picture and then multiply this
> time for the specific film I'm using for the next shot? Thanks. >>
>
>Your description of reciprocity failure is essentially correct. And yes,
the
>incredible low-light metering capabilities of the OM-2(n, S) and OM-4(T,
Ti)
>can lead to improperly exposed photos due to reciprocity failure.
>Unfortunately there is no single formula to figure out what compensation
>factor to use. It varies from film to film. To apply the compensation, I
use
>the exposure compensation dial on my OM-2S and OM-4. Best advice is to test
>with the different types of film you use, and still bracket.
>
>On a related note, I've read before that reciprocity failure can also occur
>with extremely short exposures, such as those obtained with an electronic
>flash at close distances. Has anyone ever experienced this phenomenon?
>
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