usher99@xxxxxxx wrote:
> What is required for a lens to communicate with an OM body so the meter will
> function?
>
> Seems the only communications ( lens to body) are the auto aperture trigger
> (hopeless cause on a non system lens) and the aperture-meter coupling tab
> doohicky.
Correct.
> With a Zuiko, the manual exposure meter will respond to changing the aperture
> setting on the lens even with the lens wide open. Now even my only T2 mount
> lens (Celestron 750 f8)has a small tab and it seems to meter OK manual and
> auto. With no tab the cam thinks the lens is (I think) about F32)
The camera never knows the actual full aperture of the lens. It only
knows where it is set relative to full aperture. The coupling pin on
every lens is at the same radial position at full aperture, no matter
what that aperture may be. As the aperture ring is "stopped down", the
camer reads the movement as stops down from the unknown full aperture.
> Will auto mode still work with no tab?
I believe that the TTL auto should work correctly, as is simply measures
the light on the film plane. But I have no practical evidence, as I
don't have any lenses or adapters without the aperture tab. The Sigma
600/8 has a fixed tab. The T2 adapter I use on the Meade 1000/11 has a
fixed tab as does the other one sitting in the "adapter stuff" drawer.
The various Spot auto functions depend, I believe, on the Aperture tab
being there.
> Does the tab tell the cam the aperture is F1.2? I don't understand how an F8
> lens willl meter properly. The 2nd metering circuit on auto visible in the VF
> seems to work OK with that lens. The shift lenses are essentially stop down
> metering, so perhaps just the tab is necessary.
1. No.
2. Does the above make that clear?
3. Yes, because it has the tab.
4. Yup. you got it!
> My 35 shift clearly meters fine in both modes pre shift.
>
> Why I am asking will be perfectly clear in a few weeks.
>
Whatever you are doing, it should end in an OM mount with tab. There are
several possible sources. T2 mounts, extension tubes. cheap, 4-element
teleconverters, broken, fungusy, lenses, crap 3rd party lenses, etc. are
all potential sources. For those with movable tabs, epoxy or a little
screw would fix them.
For something with a short mount of its own, the adapters for various
small reflector telescopes provide a longish tube that could be cut to
length and adapted to all sorts of things, and which has a T thread for
T2 adapters on one end. That's probably how your Celestron is attached
to cameras.
> Thanks for any comments. I can't think of any better place to ask, but Fri
> PM is likely not the best time.
>
> A. Confused Mike
>
I'm confused about many things, and can mostly appreciate that as a
natural, maybe even a good thing. Thanks for asking about something I
know a bit about. :-)
Moose
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