In article <[email hidden]>, GSV Three Minds in a Can
URL: said:Bitstring <[email hidden]>, from the wonderful
person W. D. Grey <[email hidden]> saidQuoted message said:In article <[email hidden]>, Paul Saunders
<[email hidden]> writesQuoted message said:Actually it's a pain in the [censored], especially with reciprocity to screw
things up.Go on - your dying to tell us. What's reciprocity ?...:-) :-) :-)
DON'T YOU DARE !
He may not, but I do. It's the problem where doubling the exposure when
the light level haves (which is what you'd normally do) quits working at
very low light levels. You wind up having to quadruple the exposure time
(or worse) instead.Reciprocity = 1/N (the reciprocal, see) .. except it isn't, at very low
levels, hence reciprocity failure. See, walking is very educational. 8>.
It's as a result of the fact that each grain in the film requires
several photons within a given window of time. You can to some extent
modify this behaviour by pre-flashing the film with a low level of light
evenly over the whole film such that each grain only needs one more
photon to respond. However if you overdo it you fog the film. [I used to
have to do this for DNA sequencing when I was running radioactive
samples on gels and then exposing the gels onto sheets of film. These
days its done using florescent dyes and passing the samples down a
column in front of a detector and photomultiplier.]
Mike <URL:http://www.path.cam.ac.uk/~mrc7/>
--
o/ \\ // |\ ,_ o Mike Clark
<\__,\\ // __o | \ / /\, "A mountain climbing, cycling, skiing,
"> || _`\<,_ |__\ \> | immunology lecturer, antibody engineer and
` || (_)/ (_) | \corn computer user"