Pre-fogging film doesn't really increase its sensitivity. It compresses the
brightness range. In photographing galaxies and bright nebula very often
the center portion is very dense and bright but the outer regions are too
faint to record well. By pre-fogging the whole frame gets a "boost" above
base density + fog, moving the toe of the sensitivity curve up to a more
useable region. Then any additional detail of the object will record where
it's useful. The effect is most noticable in the most faint areas; the
effect in the brightest regions is minimal.
This same principle applies in the darkroom when print high contrast
images - Cibas in particular. The pre-fog kicks the shadows up just enough
to reduce blocking, then the "real" exposure is reduced a tiny bit to
preserve the highlights.
John P
______________________________________
there is no "never" - just long periods of "not yet".
there is no "always" - just long periods of "so far".
Tim Hughes <HI100@xxxxxxx> wrote:
> isn't the point here that you had "prefogged" (actually normally
> exposed) the film and hence increased it's sensitivity to the X-rays? I
> believe prefogging is commonly done by astro-photographers to improve film
> sensitivity. It reduces the film's non-linearity (reciprocity problems) at
> low light/em radiation levels.
< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
|