Archived posting to the Leica Users Group, 2002/01/13

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Subject: RE: [Leica] NEW M7 MAJOR CROCK-POT CROW PARMESAN
From: Henning Wulff <henningw@archiphoto.com>
Date: Sun, 13 Jan 2002 17:51:06 -0800
References: <NEBBJDFBIKOBILIKPPBNEEAFIEAA.red735i@earthlink.net>

At 2:41 PM -0800 1/13/02, Frank Filippone wrote:
>Doesn't OTF mean OFF THE FILM... implying the shutterr could be painted blue
>with purple spots for all it cares?
>
>Frank Filippone
>red735i@earthlink.net

Yes, but.... In high speed flash (high shutter speed, ie, above the 
'sync' speed where the shutter is completely open all at once, the 
flash has to be read off the shutter curtains _and_ the film through 
the slit that is open for the flash calculation to work, so whatever 
part of the curtain is still blocking the film at that time has to 
have similar reflectance as the film. All the little pulsed flashes 
have to have the same intensity, so I imagine the flash exposure 
calculation has to take place during the first flash, so that it can 
get quenched properly, and the subsequent flashes can get quenched to 
the same degree.

That's somewhat tricky, and that is probably why Canon, for one, does 
a preflash milliseconds before the exposure, and bases the flash 
output on the measurement of the preflash, so that the effects of 
variable slit openings and thus variable coverage by the curtains 
during high speed flash don't have any influence on the calculated 
exposure, and also so that matrix metering can be used.

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    *            Henning J. Wulff
   /|\      Wulff Photography & Design
  /###\   mailto:henningw@archiphoto.com
  |[ ]|     http://www.archiphoto.com
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In reply to: Message from "Frank Filippone" <red735i@earthlink.net> (RE: [Leica] NEW M7 MAJOR CROCK-POT CROW PARMESAN)