How do the readings in the finder of the camera compare between the 85mm to
the 100mm? They should be nearly identical. If not, the aperture coupling
mechanism on one of them may not be operating properly.
If the readings are really far off, turn the aperture ring on the lens and
watch the tabs on the rear of the lens -- one should move back and forth.
If not, the coupling mechanism isn't aligned properly.
If the readings are just a little off (no more than a stop or so), someone
may have tampered with the coupling ring's positioning adjustment on the
aperture's cam. To get to it, you would have to remove the front lens
group.
It all that looks OK, someone may have tampered with the aperture's size
adjustment. Hold in the stop down button on the mount while you look
through the lens with it set wide open -- you should not see the blades move
into the optical path. Then, when you move the aperture ring one stop down,
you should see the blades move into the opening. Then, moving to the
smallest aperture, the opening should not be excessively tiny. This
adjustment is also accessed by removing the front lens group.
Next, set the smallest aperture on the lens and move the stop-down lever on
the rear of the lens to stop the aperture down and release it -- the
aperture should close and open almost instantly. If not, the aperture
mechanism is likely contaminated with oil (rare on the 100mm's, but always
possible). Only a strip-and-clean will do.
Lastly, try shooting some shots in Auto -- no matter what the aperture does,
the OTF exposure control should compensate.
>
> OM4Ti, in manual mode. Exposure from a gray chart. Metering done in
exactly the
> same way for both lenses. So something is definitely wrong with that lens.
>
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