"Secondly there were two notes against "Other Olympus Zuiko
Interchangeable Lenses," the second of which was "Use of the Zuiko 600mm
and 1000mm lenses is not recommended [with the OM2000] because a portion
of the image area will not be visible in the viewfinder."
This rather cryptic remark refers to "mirror cutoff" -- something which
I haven't heard of in years.
It's caused by the mirror being shorter than it has to be to intercept
all the rays of light from the lens -- either because the manufacturer
is cheap, or wants extra lens clearance when the mirror swings up.
This causes problems with long lenses. The rays from their exit pupils
are nearly "parallel" (as opposed to the acute rays from a wide-angle
lens) and thus "see" the full surface of the mirror. If the mirror isn't
long enough, the upper part of the image will be cut off.
The same effect could occur when using a bellows at high magnifications.
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