My understanding of flash sync is that:
- Up to a certain speed – apparently 1/250-1/500 for focal-plane shutters – the shutter curtains have a moment during which they are fully open. The flash merely needs to fire sometime during the full-open period for a good exposure.
- Above this speed the rear curtain begins closing before the front curtain is fully open, creating a slit of some width running across the sensor. High-speed-sync (HSS) flashes can still create a good exposure by firing multiple times during the shutter traverse, but that is limited in practice by flash recycle speed to under 1/1000.
But now we're into the realm of flash duration, so can't we just run the shutter while the flash is "fully open?" My understanding is that typical full-power flash durations are on the order of 1/1000 or even slower, so for these faster shutter speeds isn't it sufficient for the shutter to traverse while the flash is firing? Or is the "full-on" period during which the flash is emitting a consistent color and power much more limited?