10

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?

6

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.

11

In principle, your rationale is correct. However, there is no usable period during which an ordinary single flash is emitting at constant power.

The power of a typical on-camera flash quickly increases from zero to its maximum value in about 0.1 ms (i.e. 1/10 000th s). Then it exponentially decreases with a half-life of roughly 1 ms; i.e., it decreases to half its maximum value after about 1 ms (i.e. 1/1000th s) and to quarter its maximum value after about 2 ms. (At lower power settings of the flash, the curve may be cut off early.)

That is why a high-speed sync flash setting fires repeatedly at roughly 50 kHz (i.e. 50 000 pulses per second). Thus, the individual pulses overlap and create an approximately constant light source.

1

I think a key complication with doing it this way round is going to be getting the flash to be able to synchronise with the shutter at that level of precision. Eg, what happens if there is a long sync cord, or a radio trigger? They may cause different amounts of lag (I realise that the lag with a longer sync cable will be negligible and may not make a difference).

It is easy to get a flash to fire within a window of 1/200 second, but as @SailorCire mentioned in their comment, the amount of power required prohibits the length of time the flash can be lit, so this window is so much smaller (and so much more variable depending on which flash gun...which also has to include studio strobes etc as well)

  • Xenon flash-tubes dump the full charge of the capacitor in a short time, though that time could be extended with an inductor in series. It should be possible to configure the duration of an LED flash, but I did not find any information on that. – DrMoishe Pippik Apr 7 '15 at 15:40
0

Shutter curtains/blades move at a constant velocity.

Doesn't matter how fast your shutter speed gets, the time it takes the exposure to happen never gets faster than about half your sync speed. That is, about 1/500th on a 1/250 sync speed shutter.

Past sync speed you're just making the gap between curtains smaller. 1/8000th on a 1/250th sync speed camera still takes 1/500th of a second to happen...but you're only exposing 1/16th of the frame at any given time.

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