Nowadays Olympus' repair manuals have a section called "Restrictions" --
things like, "If customer uses film without DX coding on the cassette, thus
and such will happen", or "With the back open but the back latch closed, the
other will happen", and so on. Back when we trained on the OM-4, they
didn't have such a thing -- the trainer just read them off to us item by
item. This is one of those cases....
We were warned that, due to the optical characteristics of unusually wide
angle lenses, the reading in "Spot" mode would vary, and there was no means
to correct for that -- end of discussion. I was able to pry out of the
trainer that it had to do with the angles at which light exited the rear of
wide angle lenses, reflecting off the sub-mirror, thence into the SbC array.
Thomas Müller wrote:
> Thanks Winsor for your kind reply.
>
> I did not exchange the focussing screen. The OM4 has the 2-4, the
> OM4Ti the 2-13. I was just speculating if that might be the eason
> that they behave differently.
>
> Yes, I used a wall as well. Same thing. With the wider lenses the OM4
> showed one Stop difference between center-weighted and Spot reading.
>
> Any more suggestions? Maybe a CLA is advisable?
>
> Cheers, Thomas
>
> >
> >Date: Thu, 16 Jan 2003 12:39:50 -0800
> >From: Winsor Crosby <wincros@xxxxxxxxxxxxx>
> >Subject: Re: [OM] advice needed: SPOT meter reading differs from
> >AUTO on gray card??
> >
> >>Hello Ladies and Gentlemen,
> >>
> >>a Happy not so New anymore Year to all of you.
> >>
> >>By chance I noticed something with my OM4 (focussing screen 2-4)
> >>which puzzles me: I took a reading from gray card in AUTO (center
> >>weighted average), then pressed the SPOT button, and the little
> >>diamond jumped one full Stop to the right (double exposure time).
> >>The compensation dial was set to Null.
> >>
> >>I did some more testing: This happens mainly/only with the 21f3.5 and
> >>24f2.8, but not with the 28f2 or 50f1.8 (this selection is based on
> >>lens availabilty). I also tried my OM4Ti/2-13 focussing screen with
> >>these lenses and with all of them there was no difference between
> >>center-weighted and spot reading. And with both cameras and with all
> >>the lenses the center-weighted AUTO reading was identical and precise.
> >>
> >>Does anyone have an idea what is going on? Does the combination of
> >>2-4 focussing screen and very wide lenses cause this difference? Is
> >>there something wrong with my camera?
> >>
> >>I would appreciate your input!!!
> >>
> >>Thanks and best wishes,
> >>
> >>Thomas
> >>--
> >
> >The focusing screen does not have anything to do with the metering in
> >the OM4. The light being measured is passing directly through the
> >mirror and being reflected down by the secondary mirror toward the
> >sensor in the bottom of the mirror box.
> >
> >If you found variations that disappeared by changing screens your
> >methods are suspect. I think that if you carefully recheck the
> >variation will have disappeared. Uniform illumination of the field
> >is necessary to make this kind of comparison which is hard to do with
> >a gray card. It would have to be very large to fill the field of a
> >21 mm lens and still be in focus. I would suggest that you use a
> >uniformly lit wall.
> >- --
> >Winsor Crosby
> >Long Beach, California
> >
> >------------------------------
>
> --
> Dr. Thomas Müller
> Max-Delbrück-Center for Molecular Medicine
> AG Medical Genetics
> Robert-Rössle-Str. 10
> 13125 Berlin
> Germany
>
> phone: 49 30 9406 2842
> fax: 49 30 9406 3449
>
> < This message was delivered via the Olympus Mailing List >
> < For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
> < Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
|