I also used to have one of these Lentar monsters but now have a Tamron that
I'm pretty happy with. But as to exposure problems, mirror lens are sometimes
described not by f stop by by "T stop" which is an index of light
transmission rather than true aperture. After all there is a central mirror
that blocks part of the light. Additionally the silvering on both of the
mirrors does not transmit 100% but, depending on the efficiency of the
coating and the age of the coating, each of the mirrors may transmit 850r
so. Soon you are downn to a true F11 rather than f8.
Another factor may be the T mount you are using. Look at the position of the
aperture indexing lever comparing a Zuiko and the T mount. At full aperture,
the position of the fixed aperture indicator should be in the same position
as the lever of the Zuiko at wide open aperture. If it is not, it may be
telling the camera body that it is stopped down when it is really not, which
would tell you to lengthen the shutter speed.
A better comparison between your lens would be to point them at the open sky,
or at least an evenly lit wall rather than an object in a room.
Hope this helps.
Warren
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