Archived posting to the Leica Users Group, 2002/01/13
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]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 - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html