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Re: (meteorobs) Long Exposures/Reciprocity



> Date: Wed, 16 Apr 1997 11:17:08 -0400 (EDT)
> From: GeoZay@aol.com

> While making a long photographic exposure, most of the exposure
> occurs during the first several minutes. This is because whatever
> the film speed is(ASA), it's effective speed decreases during long
> time exposures. This is reciprocity failure.  ...

This is not how reciprocity failure behaves.  Think about it this way;
because you are taking a long exposure, with fewer photons per unit
time, instead of collecting X photons, as you would for a photo at
1/60th second, you now have to collect 2X photons.  It's not that
"most of the exposure occurs during the first several minutes", but
rather the exposure happens even more slowly.  The underlying physical
effect is that it takes a finite number of photons to trigger the
chemical reaction in the emulsion of your film.  This number is
greater than one (1).  For sake of arguement lets assume the number is
5.  In the first minute of exposure, our molecule of interest absorbs
2 photons, then during the next minute it encounters none, and in
addition it re-radiates a photon.  If this sequence continues, the
molecule would be exposed in 8.5 minutes, but without the re-radiation
it would be exposed in 4.5 minutes.

> The question I'm leading up to is this: The other night while making a long
> exposure of the comet (40 minutes), I saw an airplane heading for the comet
> after about 20 minutes of the exposure. Not wanting the blinking lights of a
> plane in the picture(hoping to not get 39 flashes of light), I covered the
> 200mm lens with a piece of black cardboard for the planes duration..about 30
> seconds. Now when I took the black cardboard away, does the film start out
> again as if it was 800 ASA again? If so...could I do this every 10 minutes
> for about 30 seconds to increase the overall effectiveness of the film for
> long exposures? 

No.  In fact, you should probably figure in the "dark" time when
computing the reciprocity failure exposure.  So this would not make
matters better, it would make it worse.

Gregg Lobdell                             Voice: 206-234-0884
Boeing Commercial Airplane Group            Fax: 206-234-5775
Seattle, WA, USA                       InterNet: gml@atadot ca.boeing.com

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