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Re: [OM] Auto Exposure on OM4T(Long)

Subject: Re: [OM] Auto Exposure on OM4T(Long)
From: frieder.faig@xxxxxxxxxxxxxxxxxxxxxxxxx
Date: Thu, 25 Apr 2002 15:11:33 +0200
On Wed, Apr 24, 2002 at 07:06:30PM -0700, Winsor Crosby wrote:
> 
> 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.

I´d say:
The meter starts reading when the mirror is up, and the first curtain started.
The OTF-metering system was only possible when fast, very fast
metering systems were available (silicium cells necessary).
It is even fast enough to trigger the flash after a 1/40000'th second, when
it is enough light. So I dont worry about 1/125, 1/250 ....

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

It is always a mixture between both.
> 
> 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?  

Film reflectance is not so much different between films. Today 
OTF-flash-control  is state-of-the-art-technology. With flash,
 1000f the reading is done on the film. 
When film refectance would change, not only the OM4 would be 
troubled, all modern cameras couldn`t handle flash anymore.

Film reflectance is a issue for testing guy`s but not for practical work.
The difference it nomimnal and real film-sensivity already outweight
 this problem. 


Frieder Faig

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