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Re: (meteorobs) Long Exposures/Reciprocity
GeoZay@aol.com wrote:
>
> 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
HI George,
The film carries on where it left off.
I have never done really long exposures as I have found that you can
get the same results in 20 min.
This is so, unless you are using colored filters, which will affect your
exposure times, with the diff filters.
Some films are green sensitive(3M) red sensitive (Kodak) and blue
(Agfa). So you have to know the characterisics of the film.
Saturation of standard film occurs after about 5 min.
The longest exposure I ever took was 45 min 50mm @f22 of Orion.
This was to get the Barnards loop around Orion.
I do not know sensitising of film at all, so I cannot comment on it.
Cheers and keep well
--
Bill Hollenbach Cederberg Observatory ah52@solo.pipex.co.za
http://aztec.co.za/exinet/travel/misc/cederobs.html
http://www.cs.uct.ac.za/~iwebb/obs/
Lat: 32:30.11' (S) Long: 19:15.20' (E) GPS POSITION
" We are stardust "
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