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[OM] Auto Exposure on OM4T(Long)

Subject: [OM] Auto Exposure on OM4T(Long)
From: Winsor Crosby <wincros@xxxxxxxxxxxxx>
Date: Wed, 24 Apr 2002 19:06:30 -0700
I hope someone can clarify this for me. The way I understand OTF autoexposure on the OM4T is that once the shutter button is tripped and the mirror moves out of the way, the two part meter in the bottom of the mirror box reads light reflected from 1st curtain which then opens to uncover the film. At 1/60 sec. the entire frame is uncovered and a center weighted light reading is taken from the film. Depending on the light level and camera settings the second curtain is released when the film receives enough light for an exposure. So, apparently, at 1/60 of a second and slower a center weighted reading can be taken from the film.

However, for faster speeds, the 2nd curtain has to be released before the 1st curtain has uncovered the frame. The shutter speed is fixed once the second curtain is released. Would the off-the-film metering feature only apply to speeds at 1/60 sec. and slower? At faster speeds the meter sensor would have to be building a center weight reading reflection from both the patterned1st curtain and the film in order to release the second curtain. At higher speeds, such a 1/1000 sec. over 900f the light reflected into the meter lenses would come from the shutter curtain, not the film.

At 1/125 second the 2nd curtain has be released when the 1st curtain is half open. I am not sure how long it takes for the chip to decide that the light level and then release the 2nd curtain. The 1st curtain may not even be open half way when the meter reading is made.

It may even be that all speeds higher than 1/60 sec. are determined by the reflection from the shutter curtain.

If either case is true this has some practical considerations to speculations that have come up in the past. One would not have to worry especially about a bright area at the leading or trailing edge of the frame closing the second curtain too soon since the reading is center weighted and the lens over the meter cell at the bottom of the mirror box cannot distinguish between reflectance from film or a shutter curtain.

Second is the discussion some time ago about the effect of film reflectance on the meter reading. In spite of wide ranges in reflectance has anyone had problems with consistent exposure problems with some films? If not I suspect it is because the reflectance from the 1st curtain takes such a large part in the exposure.

Any thoughts?



--
Winsor Crosby
Long Beach, California

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