You don't say which flash and camera you have. That will be very important. Read your flash manual about high-speed sync. The problem is that, at shutter speeds over 1/200, the shutter on a DSLR is never completely open. The shutter is really two curtains that travel vertically (the short dimension of the frame). Before the shutter release is pressed, the first curtain is closed and the second curtain is open. When the release is pressed, the first curtain starts down (or up?) across the frame. The second curtain follows after the selected shutter speed. At speeds above 1/200, the second curtain starts across BEFORE the first curtain has completed the course. In essence, a strip of the image is being exposed. When the flash fires, it will expose only a strip. Some flashes can get around this by firing the flash many times, very rapidly, as the two curtains trave across the sensor.


I can help if you have a Canon 580EX or a Sigma 500 series flash.