>
> I want to add that the photographic emulsion is in fact at the back of the
> film, so it would be even more accurate to have pieces of film between the
> screen and the film rails.
OK, maybe I'm misunderstanding what Thomas means here, but since I've waited
a day for someone else to respond, and no one has, here goes. With every
camera I've ever used (aside from some special graphic arts applications),
the first thing the light strikes in the camera is the emulsion. This
accounts for the discussion some months ago regarding the reflectance of
different emulsions as affects our OTF exposure metering. Older films I
vaguely remember even had an opaque anti-halation layer on the opposite side
of the base from the emulsion that made for some real fun during processing.
I believe that movie film still has this feature, apparently rendered
unnecessary for our films.
Thus, if ground glass is spaced off the film rails by the thickeness of a
film base (and what kind- there are several different base materials,
resulting in different thinkensses), you will indeed have a focus problem.
> BTW, the same goes for knife edge focusing in astro photography, wich was
> mentioned here the other day: Always account for film thickness!
>
> Regards,
> Thomas Bryhn
Perhaps there is something different going on in astro photography, a
subject about which my cat knows more than I.
Bill Pearce
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