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Re: (meteorobs) Long Exposures/Reciprocity

In a message dated 97-04-16 13:22:01 EDT, Gregg wrote:

<< 
 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.<<

Gregg, I think I said essentially the same thing, but in a very short cut
manner.  Your explanation is very good though. 

I guess with preflashing, the film emulsion has acquired a sufficient amount
of photons to overcome the eventual loss over the long haul? 
 
 > 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.
  >>

I understand now...thanks for the explanation.
George Zay
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