In a camera, flash synchronization is defined as the firing of a photographic flash coinciding with the shutter admitting light to photographic film or electronic image sensor. It is often shortened to flash sync or flash synch.
In mechanical cameras, the synchronization mechanism usually consists of an electrical contact within the shutter mechanism. In electronic digital cameras, the mechanism is usually a programmable electronic timing circuit, which may take input from a mechanical shutter contact in some cameras. The electrical connection will be either by means of a cable with a standardised coaxial PC (for Prontor/Compur) 3 mm connector, or via contacts in an accessory mount (hot shoe) bracket.
In general, faster flash sync speeds, usually rated in fractions of a second, are better if the photographer needs to flash-fill subjects that are backlit and wants to avoid motion blur, or wants to decrease depth of field when using a large aperture.
Cameras designed for use with flash bulbs generally had one or more of M (medium) sync, F (fast) sync, or FP (focal plane) sync, designed for use with corresponding bulb types. These sync modes close the contacts a few milliseconds before the shutter is open, to put the main pulse of light at the best time relative to the shutter opening. FP sync was designed for use with FP (flat-peak) flash bulbs which were designed for use with focal-plane shutters. Most standard flash bulbs used M sync.
The Nikon F offered FP, M, and ME bulb synchronizations, in addition to the X sync.
X (xenon) sync is a mode designed for use with electronic flash. In this mode, the timing of the contacts coincides exactly with the full opening of the shutter, since xenon flashes respond almost instantly.
Due to their construction, focal plane shutters as used on most SLRs will only allow xenon flash units to be used at shutter speeds slow enough that the entire shutter 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 (e.g. Nikon D70). Electronic shutters used in some digital cameras do not have this limitation and may allow a very high X-sync speed.
Leaf shutters, which are generally much smaller and situated within the lens housing, can move much faster than focal plane shutters and allow flash sync across all shutter speeds (up to 1/1000 with a Rollei PQS lens).
Higher sync speeds are useful as they enable the brightness of the background to be controlled while maintaining normal flash exposure in the foreground, for example with a back-lit subject.
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. Only certain camera and flash combinations support this feature, and the camera-flash pairings are almost exclusively from the same manufacturer, the first being the Olympus OM-4 with the F280 flashgun. Off-camera flash units with this feature are currently very rare.
Some modern electronic cameras include the ability to fire the flash just before the closing of the shutter, so that moving objects will show a streak where they came from and a sharp image where they were at the end of the exposure, useful for moving objects to convey a sense of speed. This mode is called either rear-curtain sync or 2nd-curtain sync.
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.