High speed sync at 1/160 ?

[MariusK]

Shooting with 6d and 600ex rt

I like to use little bit of fill light when there's a bad light outdoors and since i dont want to compromise dof I do it in high speed sync mode with fast shutter and low dof.

When the action switches to indoors I allways set the shutter speed to 1/160.

The question is: Do I need to switch off thigh speed sync on the flash to preserve battery/not overheat or does it switch to regular firing automatically once i set the shutter speed to 1/160?

Just trying to minimize the amount of stuff to worry about when I shoot in hectic scenarios.

Thanks for help.

[TCampbell]

You probably want to disable high-speed sync (HSS) while shoot at or below 1/60th for your camera.

When HSS is enabled, the tube pulses the flash rapidly so that as the shutter sweeps across the sensor (exposing only a gap) it behaves like a continuous light.  But the trade-off is that it has to reserve enough power to pulse many times... it can't fire at full power.

The flash is only able to provide light at about 1/6th the power of what it can do at full power (you lose about 2.5 stops).

It's my understanding (and I have not tested this) that the high-speed pulsing of the flash does not change depending on your shutter speed.  If you use 1/160th or lower you can use normal flash.  But if you use 1/200th or faster ... all the way up to 1/4000ths (for your camera) it doesn't matter what speed you pick, the rate of the rapid pulsing of the flash is the same and you lose 2.5 stops of light (unless you cluster flashes).  According to Syl Arena... the rate of the rapid burst is about 35,000 times per second!

I don't think Canon documents this.  Syl Arena (author of Speedliter's Handbook ... a VERY good book on the Canon Speedlite system) says that when he was at or below flash sync speed but the flash was in HSS mode, he noticed no drop in power output.  In other words it behaves as if it knows it doesn't need HSS and just does a traditional single burst of flash.  As soon as he set the speed to anything faster that the flash-sync (often abbreviated X-Sync) he noticed the 2.5 stops of drop in light.

This seems to suggest that the camera is smart about it... even though I don't see any specific documentation to confirm that.