Does the DF+ sync speed depend on aperture?

[siddphoto]

I was using a 645DF+, with Credo 40, and 28mm and 55mm LS lenses.
Light was a Profoto B1, triggered by a Profoto air remote.

I was able to get it to sync at 1/1600 if the aperture was 5.6 or brighter. f8 onwards, I wouldn't get the power form the flash if I shoot any faster than 1/500 (or, 1/640). Is this normal?

[Christoph B.]

Well the B1 is a fast flash with a short duration on low power settings in freeze mode (1/19,000) - but with full power (500W/s) it only gets up to 1/1,000s on t0.5.

That means 50% of its power is delivered during 1/1000s, the rest follows later. The actual important time is t0.1 during which 90% of the flash power is delivered but t0.1 is about 3x longer than t0.5. That's why manufacturers like to boast with t0.5.

So basically the B1s actual flash duration is between 1/6333s and 1/333s.

Therefore if you're shooting at 1/1600s and your flash is set to a higher power setting it will take longer to fully expose than your shutter will allow it to - the shutter cuts off the flash while it's still firing.
Likewise if you're shooting at 1/500 at full power it will still be cut off, as the flash takes 1/333s to fully expose the image but your shutter won't allow it to.

http://strobist.blogspot.co.at/2010/06/rise-and-fall-of-machines-understanding.html

[siddphoto]

Yes - sorry I should have mentioned. I kept the flash power at 1/8th or so that the flash duration is short enough.
Keeping the shutter speed at 1/1600, and going from f/5.6 to f/6.3, there is a drastic change in total light gathered by the camera. I was not sure if this is normal or not - hence asked.

If this is not normal, I'll take some more test shots, and post examples.

[Christoph B.]

could you post the images or examples with details like shutter speed, flash setting, aperture, camera setting,...?

[Ken Doo]

Just checking----did you set the Profoto Air Remote to "fast" mode?

[aztwang]

Just checking----did you set the Profoto Air Remote to "fast" mode?

+1 Ken. You will loose some light if your transmit speed is slow.Also make sure the B1 is set to speed mode. (I think thats what they call it.. ) That being said I don't think I have ever seen a large difference in exposure from 11/3 of a stop.