ScottGee1 wrote:
>While making some low light pix with my 4T in Auto mode, I noticed
>that even when the display indicates 1/2 second, the shutter responds
>according ONLY when I spot meter. Otherwise, the shutter stays open
>much longer.
>
>To eliminate the obvious problem of spot vs. centerweighted bias, I
>pointed the camera at an evenly lit wall. Set f-stop to get 1/2
>second exposure on the internal display. Press spot, release shutter,
>sounds like 1/2 second. Same thing, this time w/o pressing spot and
>the exposure is 2.5~3 full seconds. It's also proportional, i.e., if
>the meter display reads 1 second, spot metering seems correct and
>after the centerweighted reading the shutter stays open even longer.
>And proportionally shorter as well.
>
>
You are dealing with 2 completely different exposure systems here. In
Auto Mode, the camera reads the light falling on the first curtain and
the film until enough light has accumulated to complete the exposure
(slightly simplified). In Spot Mode, there is no way to do TTL, even
with only one spot, because the camera has no way to know if the spot
metered is still in the same place in the frame. In Multi-Spot, it is
even clearer that exposure cannot be done TTL. So in Spot, the exposure
set in the viewfinder is set and used as a fixed speed when the shutter
is released.
So what is happening is that the two systems disagree about the correct
exposure.
>Am I doing something wrong?
>
Very likely, and if so, that's the good news. If you don't have film in
the camera while you are playing around, you will get this effect. The
TTL meter is calibrated to read based on the average reflectance of film
and the pattern on the first curtain; at the speeds you are testing,
mostly the film. When you use the camera witout film, the system reads
the reflected light off the all black pressure plate, thus giving longer
exposures. To play around with any of the OM bodies with TTL exposure,
you must put a roll of junk film in to get correct results. The cameras
all came new with a little piece of gray cardboard in the film plane,
but they are long gone for most bodies.
>Missing something obvious?
>
Perhaps the above.
>Or does the camera need to go to the doctor?
>
>
Unless the above is the problem, yes. You could determine from test
exposures and/or another meter known to be accurate which reading is
correct and make appropriate exposure compensation when using the other.
but basically, you want them both to be accurate and more or less agree,
given their different fields of view.
Moose
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