Hi John, Moose, Lama, Rod et al:
>If the average is correct and, as I understood Carlos' post, the spot
>mode gives a longer exposure than average mode
Yes, that is correct. The average reading remains consistent within
different lenses, and matches closely other bodies' readings -- it's the
spot metering the one which shows variations.
>His camera is likely not correctly adjusted
This morning I repeated most tests in a different setup (much higher light
level, 1/125 @ f11 or so), with both the OM-4 and the OM-2S.
Both bodies got similar results, between them and also compared to my
previous tests. Sometimes the -2S gave spot meterings a bit closer (just
1/3 EV) to the centre-weighted (or "average") readout, though.
The most significant difference was with the 35/2.8 and 55/1.2. Both bodies
agreed on a 2/3 EV offset, instead of the full stop of the former test --
not that different.
>it is actually
>surprisingly difficult for the human eye to tell if a 'uniformly' lit
>subject is actually uniformly lit within a stop or 2
Sure it is, but for each lens I took *several* readings, both CW and spot,
in different positions, horizontal vs. vertical, etc, just to discard any
unevenness. Sometimes there was a slight (1/3 EV) difference, but it
affected both readouts (CW and spot) by the same amount.
The only doubtful cases were the ">1.1M" Zuiko 50/1.4 and the Tamron 28/2.5
-- the offset of the first going between 2/3 and 1 EV (more consistently
2/3 on the second test), whereas the Tamron was between 1/3 and 2/3 (quoted
'-0.5' in my previous post). Once again, differences up to 1/3 EV, but no
more.
>But let me ask you this: Did you examine the meter reading using the
>averaging meter each time you switched lenses?
Yes. As previously stated, I usually took 4 to 8 readings for each lens --
half in average, half in stop. With the OM-4 it's quite easy: remember CW
metering (maybe after adjusting aperture to get a comfortable readout) ->
press 'Spot' -> check offset -> press 'Clear' -> change camera position,
and back again...
With the OM-2S it's a bit trickier: set a suitable shutter speed (usually
1/125 in my last test) -> turn to 'Manual/Spot' -> adjust aperture to get
"correct" exposure -> set to 'Auto' -> check new indicated speed against
the manual setting -> change camera position, and back again...
>and have never seen this problem in practice (a 1.7EV underexposure would d=
>efinitely show on the slides).
As said before, the offset leads to overexposure -- definitely MORE
noticeable on slides!
My first OM body was in fact the OM-4. I got quickly used to the spot
metering, but my lenses so far were the 35/2 and the Tamrons 90/2.5 and
70-210/3.5 (mostly used at the long range) -- in other words, all on the
'minimum offset' group.
A few months ago I took the OM-2S with the 28-48 and slide film. I didn't
took exposure notes, but most shots were at the 28mm setting and I used the
Spot mode a lot (dangerous combination, see my testing -- 1.7 EV offset!)
A bunch of those slides look overexposed. Since I hadn't used the Spot
meter for a while, I thought I had lost my skills on spot-metering... but
now I've got a different reason!
>1. average indication (using the fresnel mirror, value not stored)
>2. CW exposure (off the film, actual light read durng the exposure)
>3. spot indication (using fresnel mirror, stored into memory)
>4. spot exposure (using memory only, light is not measured during exposure)
>
>What matters is "2" and "4".
Of course. As you said later, "1" is only a rough indication of "2",
whereas "3" is just showing the selected value for "4".
>I promptly checked that (with a piece of film =
>in the camera). Bingo! "2" and "4" agree to within 1/3 EV.
I'll try to make a test with slide film, to check real-world accuracy. I'd
be very happy with your 1/3 EV margin, but I'm afraid I won't be able to
get it with several lenses.
I plan to place a K*dak 18 0rey Card in the middle of a typical scene,
comparing manual exposure on the OM-4 (to match "1") with OTF-Auto exposure
("2") and Spot metering ("4", which would match "3") over the gray card.
>My selection of lenses i=
>s somewhat limited, so I could draw limited conclusions.
Maybe some of your lenses are included on my post. Which of them gave you
the biggest offsets?
>the practical implications of the "problem" are=
> of no real consequence for the exposure accuracy
Let's hope so -- that's why I'll try to make some tests on film.
BTW, I tried the spot meter of my C*ntax Aria with CZ lenses 18/4, 28/2.8,
35/1.4, 45/2.8, 135/2.8 and the *very same* Tamron 28-135 I tested on the
OM-4 (gave -0.7 EV then) -- no offset between spot and CW here.
However, this camera doesn't have the semi-silverd mirror and Fresnel
secondary mirror of the OM-4 and -2S. I wonder how the OM-2000 would
perform...
>and I think it is not wor=
>th losing any sleep over this.
Really smart conclusion ;-)
Thanks to everyone,
...
Carlos J. Santisteban
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