Is there no crossover point where shutter speed overtakes flash duration?

My understanding of flash sync is that:

1. 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.


2. 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?

Answer

Yes. What you're envisioning is something that's actually used by some TTL-capable radio triggers to allow faster shutter speeds with manual flashes and studio strobes: it's called tail-sync (aka "HyperSync", "Supersync", etc.).

The problem, as Loong has pointed out, is that the light/power output of the flash pulse is not even and constant during the duration. Most of the light is pumped out at the beginning of the pulse, and then it tails off relatively quickly. If you sync in the usual fashion, you'd get an exposure gradient across the frame: lighter at the top, shading towards darker at the bottom.

You have to time the sync a little later than usual, so that the light that's used is the more constant, flatter "tail" of the burst. But you're between a rock and a hard place with this kind of syncing. The duration of the pulse will only be long enough to use if you're at full power on the flash, but you're dumping most of the power release at the beginning of the pulse, so you're losing even more than the two stops you would to HSS. And this will only work for a relatively narrow combination of fast shutter speeds and high flash power (i.e., tail sync doesn't work with most flashes/strobes unless they're at full power and your shutter speed is over 1/1000s), and is completely dependent on the strobe's burst duration and output pattern (which is why a lot of those radio triggers offer ways to adjust the timing for tail sync).

See also: PocketWizard's page on HyperSync & HSS.