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.
The earliest flashes consisted of a wad of magnesium powder that was ignited by hand.
Later, magnesium filaments were contained in flash bulbs, and electrically ignited by a contact in the camera shutter; such a bulb could only be used once, and was too hot to handle immediately after use, but the confinement of what would otherwise have amounted to a small explosion was an important advance.
An electronic flash contains a tube filled with xenon gas, where electricity of high voltage is discharged to generate an electrical arc that emits a short flash of light.
In a camera, flashsynchronization is required for the firing of a photographic flash to coincide with the shutter admitting light to the photographic film.
X synchronization is when the timing of the contact coincides exactly with the full opening of the shutter.
Note that due to their construction, focal plane shutters as used on most SLRs will only allow xenonflash units to be used at shutter speeds were the entire film is open at once, typically at shutter speeds of 1/60 or slower, but some modern cameras may have an X-sync speed as high as 1/500.
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