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[OM] Re: 4T Auto confusion - display vs. shutter

Subject: [OM] Re: 4T Auto confusion - display vs. shutter
From: ScottGee1 <scottgee1@xxxxxxxxx>
Date: Thu, 7 Apr 2005 07:04:30 -0400
Moose, many thanks for making the time to explain this.  You were
correct; I was indeed shooting 'dry', i.e. w/o film.  Put a test roll
in and viola! shutter timings now are very close indeed.  Thought I
had read the manual thoroughly, but apparently didn't put the pieces
together.

Cheers!/ScottGee1

On Apr 7, 2005 2:21 AM, Moose <olymoose@xxxxxxxxxxxxxxx> wrote:
> 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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