Quote Originally Posted by Photo-John
The problem with mtb photos and high-speed sync, is you usually aren't that close. It's the limited flash range that's the problem. I always get confused when I try to sort it out. I assume the flash has to increase the output to compensate for a shorter duration. But I would think that it would actually have more reach, then. There's some detail I'm missing. I know that when you use high speed sync, you definitely have less reach, though.
It might have to do with the pulse length of the flash, which I intend to measure with a scope and a photodiode. If the flash pulse is 1/500 s long, then when you open the shutter for 1/1000 of a second, you get half the light since the flash is still spewing out light when the shutter closes again. So what you said makes sense. The reach is dependent on the amount of light and the duration. So twice the distance and cutting the light in half would require 4 times the light. The feedback on the camera measures and compensates but can't make up for the timing part.