Luca A. wrote:
>I doubt that the modern AF SLR can do that, since the exposimeter receive
>light froma semitransparent mirror. Only the flash exposimeter is exposed
>(sorry for the repetition) all the time long to light.
>
I can't speak for other brands, although I believe they work in a
similar fashion, but what you say is not true of the Oly TTL-OTF bodies.
In the case of the OM-2sp, 3, 4 and PC, a reflector below the partially
silvered part of the mirror reflects light to a sensor assembly on the
bottom of the mirror box. This provides the input for the exposure info
shown in the viewfinder. In Auto mode, the same light sensor now "sees"
light from the film plane and takes a quick reading off the first
curtain when the mirror has gone up prior to actual exposure. If the
needed exposure is 1/60 or less, the 1st and then 2nd curtains are
released to create the traveling slit to give the correct exposure. If
longer, the 1st curtain is released and the system continues to read
light off the 1st curtain and film itself until sufficient exposure is
reached, then releases the 2nd curtain.
The mirror with sub-mirror and sensor on the floor of the mirror box set
up on my Can*n 300D looks very similar and I suspect that it operates
similarly.
Roland Ruehl wrote:
>Unfortunately I do not own a 4. But I have always wondered how
>off-the-film measurement (and dynamic speeds) works together with
>multi-spot measurement. Say, for example, you have defined two spots,
>and then during a long exposure, the spots change light differently,
>what happens ? Does the camera simply perform an overall level adjustment ?
>
Spot metering on the OM3 and 4 series is TTL, that is, the measurements
are taken through the lens, but not OTF, light changes are not measured
off the film during exposure. In Spot mode, the (average) exposure from
the chosen spot(s) is calculated, a shutter speed (stepless) is
calculated and that speed is used for the exposure. This is not only
technically much easier, but truer to the purpose of spot exposure
metering. If I measure a spot nowhere near the center of the subject
frame, I don't wnat that reading/setting changed by relationship of the
overall brightness of the subject frame relative to the overall
brightness of the frame that happened to have the spot I chose to
measure at its center.
Moose
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