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Re: (meteorobs) Long Exposures/Reciprocity

> George Zay (GeoZay@aol.com) wrote:
> ... with preflashing, the film emulsion has acquired a sufficient amount
> of photons to overcome the eventual loss over the long haul? 

One of two things happen with pre-flashing:

1) Photo-sensitive molecules acquire sufficient photons so that not
all are re-radiated, but not enough to completely expose the
molecule, so at exposure time, fewer photons are required to expose
the molecule

2) There are more complex interactions happening.  Exposure happens
not only on the molecular level, but also at the "grain" level, each
grain being made up of multiple molecules.  Preflashing fully exposes
some of the molecules in each grain, so that fewer molecules are
required to be exposed by starlight to fully expose the entire grain.

Reality may be a combination of the two.

Hypering changes the chemical composition of the film in such a way so
that fewer photons are required to expose any particular grain/molecule.

> So after a 40 minute exposure where an 800 ASA film dropped down to 15 ASA
> equivalency, I can make a double exposure on the same frame during
> daylight...if I considered it as having 15 ASA?

No, for an additional exposure in daylight, you would rate the film at
800 ASA.  You might even rate it higher, depending on how you do
double exposures, and because the film may be effectively pre-flashed
by the long initial exposure.

> I can see how preflashing wouldn't change the ASA any based on what you
> say..dot since the preflashing operation is only a fraction of a second...prior
> to reciprocity failures occur.  

Pre-flashing should change the ASA, both in daylight and for time-exposure.

> But I don't understand how the film would recognize the difference
> between a short preflashing to the light it has received from a
> dimly lit object during a 10 minute exposure?

They should be the same, although the "flash" received from a
time-exposure is not uniform, (you are taking a picture of something,
right?) and it requires more photons than the short pre-flash.

Two analogies: 1) Film has good short-term memory, but lousy long-term
memory.  2) If you shout at it, it remembers, but if you whisper, you
have to whisper multiple times.

I'm not a chemist, but I play one on the net, so take all of this with
the standard grain of net-salt.  On the otherhand, I am an amateur
photographer, and have dealt with reciprocity failure in a number of
settings.

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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