Archived posting to the Leica Users Group, 2002/01/13
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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.
- --
* Henning J. Wulff
/|\ Wulff Photography & Design
/###\ mailto:henningw@archiphoto.com
|[ ]| http://www.archiphoto.com
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