Sandy wrote:
> But my point is not that practical testing in the camera is not
> relevant, but simply that it is invalid as a means of comparing EFS
> of different films. For that you need a system with a common light
> source and some control that permits repeatable exposures to an
> accuracy of about 1/10 of a stop.
In camera testing doesn't [directly] provide an absolute EFS, because some
flare light contributes to the low densities where speed is determined,
But it gives comparative results just as accurately as the BTZS method, and
that's what we usually want to know for pictorial photography -- the
difference between two films, not the absolute photon efficiency of the
film. Now, if we were making photon dosimeters....
[In actuality, one can calculate the true EFS easily enough from in-camera
results by accounting for the flare illumination -- in essence, the reverse
of what Sandy does when he corrects his shadow exposures for flare.]
Best regards,
etienne
Received on Fri Nov 12 16:50:10 2004
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