I am still experimenting daily with my external flash. Tonight I wanted to show some motion in the dune grass, but still have it look somewhat sharp so I set my shutter speed to 1/200 of a second and used the rear sync option on my flash. With rear sync, my flash went off at the end of my exposure, freezing the motion of the blowing grass. F7.1 at 1/200, ISO 100, 18-50 mm lens at 40 mm