Re: Lighting questions
- From: Alan Browne <alan.browne@xxxxxxxxxxxxxxxxxxxxx>
- Date: Fri, 06 Apr 2007 10:16:47 -0400
Robert Coe wrote:
If I'm not mistaken, preflashes are a rather recent invention. In earlier
SLR Pre-flash metering began as a means to improve the flash exposure even before digital cameras required it. I'm probably off on the dates but IIRC it began in the late 1990's.
times, you dialed the film speed into the flash unit, which monitored the
light reflected off the subject during the actual flash and decided when to
shut down. (At least that's what the literature advertised, and I had a flash
unit for my Nikon film cameras that claimed to work that way.)
That's what TTL flash is about: Stopping the flash when enough light has bounced off of the film to indicate a nominal exposure. This is done via a transistor called a thyristor which opens the circuit to the flash tube very quickly.
It may be that
modern flashes are too fast for that; in olden days, flashes couldn't be
faster than around 1/1500th of a second or the film wouldn't respond.
Not sure what you mean by "olden days" but films since WW II should respond to 1/50,000 or slower flash periods if enough photons struck. After all, they did.
Typically, however, a high end attached flash in usual conditions (f/4 - f/8, ISO 400 - 100, range of 8 - 20 feet) will output 1/8 to all of their energy and this can take 2ms or more (1/500 or slower).
The preflash doesn't have to fire with the mirror
down; it just has to fire before the shutter opens. There's at least an order
of magnitude difference between the time it takes to get the mirror out of the
way and the time it takes to fire a preflash.
What really matters is total shutter lag. The order of events is otherwise irrelevant. The major contributor to lag is the mirror. Sans mirror and a decent SLR can do about 6 ms or better. With the mirror, the fasted SLR's get about 40 - 45msec.
However, the main reason the pre-flash fires "mirror down" in most (all?) DSLR's is to use the same exposure sensors as available light for the measurement. These sensors are up in the prism area or behind the mirror (in a semi-silvered area).
The SLR OTF TTL-flash sensors could be used with the mirror up and shutter closed by perhaps adding white or grey to the lens side of the shutter curtain. (Similar to the white circle on the shutter curtain of the Leica M6 for available light metering).
OTOH, by not adding OTF sensors and doing both available light and flash pre-metering with the prism area (or behind the mirror) sensors the cost of the system is reduced. All of the SLR makers are working hard to absolutely minimize cost.
Cheers,
Alan
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