High Speed Sync

[PacAce]

Originally Posted by Jon

What happens in FP mode is the flash fires a series of less-powerful bursts over the duration of the shutter curtain's travel. These are powered and timed so there's one and only one flash fired as each part of the sensor is uncovered.

Example: When you're at 1/250 or slower, the entire sensor's uncovered before the second curtain starts to close, so ther only needs to be one burst (at up to full power). At 1/500,the flash will have to fire twice (second curtain starts closing the shutter while first curtain's only half-way through opening), once for the "top" and once for the "bottom" of the sensor. Obviously since it needs to fire twice, it can't fire full power. At 1/1000, it fires 4 times, and still further reduces power. And so on.

That's what I used to think, too, but that's not really the case. What happens in FP mode is that the flash fires a rapid burst of very short pulses and they are fired so quickly that the pulses meld into a seemingly continuous stream of light for the duration that the curtains are open and travelling across the frame.

[guitarman]

Originally Posted by PacAce

That's what I used to think, too, but that's not really the case. What happens in FP mode is that the flash fires a rapid burst of very short pulses and they are fired so quickly that the pulses meld into a seemingly continuous stream of light for the duration that the curtains are open and travelling across the frame.

Thats actually what I read as well.

[tim]

Originally Posted by PacAce

That's what I used to think, too, but that's not really the case. What happens in FP mode is that the flash fires a rapid burst of very short pulses and they are fired so quickly that the pulses meld into a seemingly continuous stream of light for the duration that the curtains are open and travelling across the frame.

Yep - the flash fires at 50KHz so it simulates a continuous light. It's helpful to think of it as "fast pulse" flash. I used it today, outside I often go over 1/250th so FP is a life saver for fill and catchlights.

More info, in the ever useful EOS Flash Bible.