On Mon, 24 Sep 2001, Chris Brogden wrote:
>That might have something to do with lens-to-film distance. Maybe the
>smaller mount doesn't need as large an opening because it's closer to the
>film plane than the K-mount? (Total guess) That being said, it happens
[...]
No guess at all. Did everybody forgot their high school physics at the same
time? The illumination from a light source fall by the inverse square root
of
the distance to that source. An object twice as far as another one
will receive only a fourth the amount of light the first object is
receiving.
That is why you must adjust exposure when using bellows. In typical use (no
bellows)
the built-in meters of cameras compensate for the difference in lens to film
distance
between infinity and closest focusing. The effect would be more pronounced
on
tele than on ultrawide (the 15mm hardly moves at all, compared to a 135).
I will speculate that the measuring point for the formula will likely show
the
distance is calculated from the center of a lens to the film plane for a
simple
one element lens, and from the rear nodal point for a compound lens.
Michel
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