SB 700 Newbie

[Mozziephotography]

A SB 700 should hopefully be delivered tomorrow and I want to take it to a race which starts around 7pm. I have a Nikon D7200 and will be shooting around 1/640 sec for the faster runners at a specific point. Looked at High Speed Sync videos on you tube. If I set the camera up for a specific shutter speed and aperture, what do I set the ISO to? Auto? Can't find anything on you tube relating to shots of runners. Sorry to be so dim.

 

[Kodiak Qc]

what do I set the ISO to?

I never worked with your specific gear but the higher the
ISO, the less hard the flash will have to work.

Another point is that at 7PM there is still daylight and that
demands less effort from the flash when you're not shoot-
ing against the Sun. As the day gets darker, there will be
more power demanded as your flash will have to take over.