On Wed, 12 Jun 2002, Tom Trottier wrote:
> On Wednesday, June 12, 2002 at 17:05, Winsor Crosby
> > I would not be surprised if all exposure determination started
> > between the time that the shutter button is pushed and the point that
> > shutter curtain opens. It would explain the shutter lag with the OM2s
> > and OM4s compared with the manual cameras. That would explain why no
> > one notices auto exposure errors at 1/60 sec. with a film with out of
> > parameter reflectivity. So I don't think there is any need to worry.
>
> I'm sure it starts then, and when enough light is seen, the 2nd
> curtain starts closing. Since the film starts becoming visible and
> continues increasing in visible proportion throughout the measurement
> period, 1/60 would have 3/4 curtain and 1/4 film reflectivity
> measured. 1/30 would be half and half.
Exposure integration begins when the first curtain is released. There's
a nice flow chart and timing diagram on pages I-47 and I-48 of the OM-4
service manual that explains all this. Section I has all the gory details
of how exposures are arrived at in the OM-4.
-mark
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