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Re: [OM] advice needed: SPOT meter reading differs from AUTO on gray ca

Subject: Re: [OM] advice needed: SPOT meter reading differs from AUTO on gray card??
From: Michael Kopp <mkopp@xxxxxxxxxx>
Date: Fri, 17 Jan 2003 14:33:54 +1300
At 23:55 +0100 16/1/03, Thomas Müller wrote:

Sheesh! I hate to perpetuate top-posting, but I'm too lazy to rearrange
everything into proper order, much less make sure the attribution is
correct ...

But, to venture a guess, I would say there's nothing wrong with the cameras
or meters.

My guess is that the meter is programmed that way. Think of it this way.

Spot metering is used for precision, center-weighted averaging is used for
general photography, most often of larger subject fields that contain a
wider variety of exposure values.

It's kind of like the difference between using a Weston meter (reflected)
reading a gray card at zone five (the indicator arrow) and reading a
Sekonic L38 (Norwood Director to us oldies) (incident) meter, the index
arrow of which is roughly zone six, or skin tone. There will be the same,
approximately one-stop increase shown by the Weston meter.

If you were to photograph a general scene, with sky, tree shadows, people,
etc., using the OM spot meter, and average the readings from medium shadow
to sky, you would probably come out with an exposure that is correct for
zone six, or the general average of most scenes. If you were to use the
spot meter and read only the medium tones of the scene, you would probably
find that it called for something like one stop more exposure than the
wider average.

Hope this helps,

Michael Kopp
Wellington, New Zealand


>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 M¸ller
>Max-Delbr¸ck-Center for Molecular Medicine
>AG Medical Genetics
>Robert-R?ssle-Str. 10
>13125 Berlin
>Germany
>
>phone: 49 30 9406 2842
>fax:   49 30 9406 3449
>
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