I'm hoping I've still got this straight and have not misremembered
anything ;-)
The 2S/PC each had a governor on the diaphragm stop down mech which
would set the "proper" f stop / shutter speed based on BV of light read
off the first curtain/film. The governor was a slow mechanism and
limited motor drive use to 3 fps.
Yes, beginning of exposure is always read off the first curtain (cell
turns on as mirror goes up).
The Program modes were not adjustable.
___________________________________
John Hermanson | CPS, Inc.
21 South Ln., Huntington NY 11743
www.zuiko.com | omtech1@xxxxxxxxxxx
Olympus OM Service since 1977
On 5/14/2014 4:33 PM, Ken Norton wrote:
I am still not sure about the OM metering off the curtain at speeds just
above sync speed. What you say makes sense, but still have a vague
recollection that John said it doesn't quite work that way.
The OM-2S, and I think the OM-40, do things slightly different because
of the program mode which activates the aperture blades at a later
stage than mirror up and is driven by a different type of mechanism
than the usual aperture stop-down mechanism in the other OM bodies.
But the way the OM system operates is that the first curtain has the
reflective pattern which is read by the OTF sensor cells from the
beginning of the exposure itself and will continue to read the
reflected light off the curtain and film surface until sufficient
exposure (with a time offset to correct for mechanical delay) and
triggers the second curtain.
As a result, the metering system ALWAYS begins to read the first
curtain and will continue to read the film surface until sufficient
exposure is attained. This is why the pattern changed in the first
generation OM-2. At higher shutter speeds, the OM-2 was much more
center-weighted than at longer shutter speeds.
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