Flash synchronization
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Image:Blitzanschluss.jpg 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) 1/8" connector, or via contacts in an accessory mount (hot shoe) bracket.
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 where 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. Electronic shutters used in some digital cameras do not have this limitation and may allow a very high X-sync speed.
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-sync.
Today, certain modern xenon flash units have the ability to produce a longer-duration flash to permit X-synchronization at shorter shutter speeds. Instead of delivering one burst of light, the units deliver several smaller bursts a time interval as short at 1/125 of a second. This allows light to be delivered to the entire area of the film or image sensor even though the shutter is never fully open at any moment. The downside is that the flash is of less effective intensity since the individual bursts are lower powered than the normal capability of the flash unit. This is an emerging technology; currently, only certain camera and flash combinations support this feature, and the camera-flash pairings are almost exclusively from the same manufacturer. Currently, popular pairings for this means are Nikon's F6, D200, D2Hs, and D2Xs cameras with their SB-600 and SB-800 flash units. Off-camera flash units with this feature are currently very rare.
The first camera to implement a flash synchronizer was the Exakta, in 1935.
Some flash units, called optical slaves, are light-triggered, and therefore require no electrical connection to the main flash unit.
