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(meteorobs) Long Exposures/Reciprocity
While making an exposure of comet Hale/Bopp the other night, a thought has
occurred to me. 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. A 400 ASA film may act something
like a 10 or 15 ASA film after several minutes. But it doesn't happen
suddenly. It is a gradual decrease until it reaches the 10 ASA equivalent.
This is why it's said that most of the exposure is within the first several
minutes. There are ways to get around this. One is to Hyper the film with a
warm bath of hydrogen gas for a period of time. Another is Preflashing the
film with a weakened strobe light. The idea here is to get the chemicals on
the film to be partially exposed. Then just needing a few more photons to
complete a granule's exposure, it becomes esier to get an exposure from a
very weak light source.
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?
George Zay
La Mesa, Calif.
32 North
116 West
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