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Re: [OM] Oh NO! -> Question on OTF metering & films.

Subject: Re: [OM] Oh NO! -> Question on OTF metering & films.
From: DBellamy2k@xxxxxxx
Date: Wed, 1 Aug 2001 20:08:27 EDT
Dear list,

"Skip Williams" <skipwilliamsom@xxxxxxxxxxx> wrote:

<< the  camera likes to have film in it to do the OTF metering.  Metering 
off the  pressure plate will often give erratic readings when running through 
an OTF-automatic metering cycle of the whole shutter speed range. >>

Right, now the question:

Does the camera measure the reflectivity of the film surface when it does the 
OTF cycle? I wondered about this since I read about OTF in about 1981. Films 
have different surface colours and possibly reflectivities, don't they?

I wondered if:
a. there is no compensation, which I thought maybe unlikely - how about if 
someone brings out a new film that is much lighter or darker in colour?
or:
b. maybe the camera measures the change in the silicon light cell reading as 
the patterned first shutter blind moves away and the film comes into view. 
This would fall off in a straight line if the film was less reflective than 
the shutter blind, for instance (if the incident light didn't change). The 
camera would then 'know' the reflectivity of the film, and then do the actual 
OTF measurement when the shutter is fully open, for the slower shutter speeds 
of course. ... ?

The other thing that marvelled me was how the camera can act _so_ quickly 
during OTF flash metering to cut off a flashgun whose flash duration is 
1/1000 second or even much less. Stunning really - that made me think, "These 
guys are _really_ clever."

As an addition, in OTF flash, I guess the camera can't get any idea of the 
reflectivity of the film beforehand, because before the flash actually fires, 
it might be dark!

Any answers anyone?

Kind regards,

Dave Bellamy.
http://members.aol.com/synthchap/

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