>> With various types of film, I too get variations in the 1-1.5 stop area,
>> but usually find that the OTF exposure is the more accurate than what the
>> viewfinder shows. (Again, different patterns).
>That's weird - how does the OTF system know what compensation to give
>for different reflectance films?
It doesn't. But the OTF method measure the light reflecting off the
entire frame, which is basically like an average reading over the entire
frame, whereas a static reading before exposure is a centerweighted or
single spot reading.
If you were shooting an image of a homogeneous surface, like a wall painted
a uniform color, then the full frame and centerweighted subset of the
fill frame would be measuring the same subject material, and you might
see the effect from variance in film reflectance.
Film reflectance is a non-sequitor though if you run tests to calibrate your
meter for each film you use. Film reflectance is just part of what goes
into the accuracy (or lack thereof) of the meter reading and when you choose
the ASA you prefer for the given film with the given in-camera meter, the
film's reflectance is taken into account (as long as film reflectance is
fairly uniform across different batches of the same film type).
Joseph Albert
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