A conventional camera shutter works in an entirely mechanical way. The shutter is fully closed before taking a shot. At the start of an exposure, the ‘first curtain’ opens, traversing the image sensor in the process. At the end of the exposure, the second curtain slides across in front of the sensor.
But at very fast shutter speeds, the second curtain starts to close before the first curtain has fully opened. That can cause a dark band in images when using a flashgun. The solution is to use ‘Auto FP’ (Auto Focal Plane) or High-Speed Sync’flash, which fires a short burst of flashes rather than a single flash.
