Flash synchronization
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In a camera, flash synchronization is required for the firing of a photographic flash to coincide with the shutter admitting light to the photographic film.
The synchronization mechanism usually consists of an electrical contact within the shutter mechanism. The electrical connection will be either by means of a cable with a standardised coaxial PC (Prontor/Compur) connector, or via contacts in an accessory mount bracket. Some flash units, called optical slaves, are light-triggered, and therefore require no electrical connection to the main flash unit.
X synchronization is when the timing of the contact coincides exactly with the full opening of the shutter. This is required for xenon flash. Note that due to their construction, focal plane shutters as used on most SLRs will only allow xenon flash units to be used at shutter speeds were the entire film is open at once, typically at shutter speeds of 1/60 (sometimes 1/125) or slower.
For, now archaic, flash bulbs, the contact may need to close earlier to allow the shutter to open at the moment of full light intensity. Special long duration flash bulbs allowed bulb flash to be used with focal plane shutters at any speed. The Nikon F offered three different bulb synchronizations, in addition to the X.
The first camera to implement a flash synchronizer was the Exakta, in 1935.