I might be wrong, but I think this is the reason of that behaviour:
The OM-1, like most other cameras, does light metering off the focusing
screen. If you look closely at a FS, you'll see a lot of concentric circles
-- I believe it's some kind of 'Fresnel lens'.
So the screen is a not-so-simple optical system, which may be sensitive to
the angle of light rays -- and sure it is. You may check it by looking thru
an OM-1 viewfinder and then turning half-way the mirror lock-up knob: the
image not only gets defocused and distorted, but also darkens noticeably.
However, if you only turn the knob slightly, you get the focus shift but no
darkening.
Since in an SLR the lens can't be too close to the film (less than 35-40
mm) because of the room needed for the mirror, the angle of light rays is
"never" too steep (max. about 30 dgrees, which may be the limit that FS's
are designed for). However, shifted lenses may _increase_ this angle, so
there's darkening and subsequent metering error.
On the other hand, cameras like the OM-2 do light metering off the
curtain/film surface -- much simpler optically. They shouldn't be affected
by incidence angle, so Auto exposure in the OM-2 is OK. I'm not sure about
OM-2S/P, OM-3 & OM-4, but they don't meter of the focusing screen, so maybe
they aren't affected, even in manual.
Let's hope I'm not too wrong ;-)
...
Carlos Santisteban
I think you have it, Carlos. Really clear, logical explanation.
--
Winsor Crosby
Long Beach, California
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