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Re: [OM] OM 4 (Auto) - Shutter speeds

Subject: Re: [OM] OM 4 (Auto) - Shutter speeds
From: Mark Dapoz <md@xxxxxxxxxxxx>
Date: Thu, 13 Jun 2002 00:12:45 -0400 (EDT)
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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