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X-syncX-sync is the correct flash synchronization between a camera shutter and the connection to an electronic unit. On some (older) cameras, there may be two settings for shutter synchronisation to the flash unit: one for disposable flash bulbs, and one for electronic flashes. The old flash-bulbs need some time to reach their full strength, and are triggered a little earlier than the shutter. Electronic flashes reach full strength immediately, and need to be triggered just as the shutter is open. The latter is the X-sync. Single-lens reflex cameras (SLR cameras) have focal plane shutters. The exposure time is determined by the time difference between the opening curtain and the shutting curtain passing the film plane. For shorter exposure times, the shutting curtain is activated before the opening curtain has completely passed the negative, making a continuous partial exposure along the negative. Electronic flash units emit extremely bright light, but only for a very short time (typically fractions of milliseconds in duration). The X-synchronisation time is reached when the shutting curtain waits until the opening curtain has just gone all the way, leaving the whole negative available for simultaneous exposure by the electronic flash unit before shutting. Some newer SLR cameras and flashes have the ability to expose the negative at all exposure times. This is achieved by making several small flash-bursts covering the whole negative in partial small exposures as the shutter opening passes the negative. When in use, this feature greatly reduces the flash's range.
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