12 would probably be fine. The idea is to get a statistically valid sample
size. 2 or 3, or worse, 1, won't do.
And to answer John Hudson, yes the measured exposure times will vary some. The
OTF exposure metering system is an analog system and it will vary a little on
each exposure. That is why I suggested a 60 s base exposure, to make the
differences observable with a hand held stopwatch..
You need to establish the average exposure time for each film reflectance to
determine if there is any significant difference due to reflectance. Suppose
that the measured exposure time varies about 10% (still a very good, reliable
metering system). You will get some exposures at 54 s, some at 60 s, and some
at 66 s. Now suppose that the real OTF exposure difference between films due
to different reflectance is 12 %. The average average exposure time for the
second roll is 67.2 s and varies from 60.5 to 73.9. If you only make 1
measurment of each film, you might get 54 s for the first film and 73.9 s for
the second. If you conclude from these two sets of 1 that the difference due
to film reflectance is 37%, you would have come up with a wrong, and misleading
answer (off by a factor of 3). The statistics are a little more complicated
than my example, but you get the idea.
How many measurements are enough? The standard answer is that more is better.
It is a fairly simple system so 12 would probably give you a good answer, but
most of the films we are interested in don't come in 12 exposure rolls. If you
have a 36 exposure roll you might as well use it. Then you won't have to
wonder if you took enough samples. On the other hand, if the exposure metering
is tighter than I anticipate, 36 won't give you a more reliable answer than 12.
I'll do the test when I get some time. I could be all wet, but I really don't
expect the difference due to film reflectance to be significant.
Gary Edwards
ClassicVW@xxxxxxx wrote:
> Why not shoot a roll of 12 instead of 36? How much more "precise" could the
> readings average out to with 36 as opposed to 12?
>
> George S.
>
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