Re: Flashing before or after exposure...

Earl and Patty Johnson wrote:
> Is it not the case that the number of photons is the only variable
> involved, regardless of whether they came all at once or a few at a
> time over many instances?

Actually besides the quantity of photons, the rate of photons is also
important. If the photon rate is too low the latent image can be
lost. This is what causes long duration reciprocity failure. It is
also why when doing multi-pop flash exposures 10 pops of the flash
(over a period of time) does not necessarily equal one pop of a flash
ten times more powerful.

If the pre-flash is done just before the main exposure (or the
post-flash is done just after the main exposure) there shouldn't be
any problem.

-- 
Brian Reynolds                  | "It's just like flying a spaceship.
reynolds@panix.com              |  You push some buttons and see
http://www.panix.com/~reynolds/ |  what happens." -- Zapp Brannigan
NAR# 54438                      |  
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