This fellow found film reflectivity differences up to 1.5 stops:
http://www.markcassino.com/essays/ttlflash.htm
Tom
On Thursday, April 25, 2002 at 11:46, Jim Brokaw <olympus@xxxxxxxxxxxxxxx>
wrote re "Re: [OM] Auto Exposure on OM4T(Long" saying:
> on 4/25/02 6:11 AM, frieder.faig@xxxxxxxxxxxxxxxxxxxxxxxxx at
> frieder.faig@xxxxxxxxxxxxxxxxxxxxxxxxx wrote:
>
> snipped...
>
> > Film reflectance is not so much different between films. Today
> > OTF-flash-control is state-of-the-art-technology. With flash,
> > 1000f the reading is done on the film.
> > When film reflectance would change, not only the OM4 would be
> > troubled, all modern cameras couldn`t handle flash anymore.
> >
> > Film reflectance is a issue for testing guy`s but not for practical work.
> > The difference it nominal and real film-sensitivity already outweigh
> > this problem.
>
> It seems to me that you would want the film reflectance to be minimal, as
> any light that is reflected from the film's surface is not activating the
> film emulsion. That may be why the reflectance is within a narrow range, and
> probably as low as can be economically achieved. If the film surface
> reflected no light it would appear a dark black (I think) and all the light
> would be absorbed into the emulsion. This would allow use of less silver, so
> the film manufacturers must have tried to get as close to the optimal as
> they can...
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