X-sync

In a camera, flash synchronization is defined as synchronizing the firing of a photographic flash with the opening of the shutter admitting light to photographic film or electronic image sensor. It is often shortened to flash sync or flash synch.

In cameras with mechanical (clockwork) shutters synchronization is supported by an electrical contact within the shutter mechanism, which closes the circuit at the appropriate moment in the shutter opening process. In electronic digital cameras, the mechanism is usually a programmable electronic timing circuit, which may, in some cameras, take input from a mechanical shutter contact. The flash is connected electrically to the camera either by a cable with a standardised coaxial PC (for Prontor/Compur) 3.5 mm (1/8") connector (as defined in ISO 519), or via contacts in an accessory mount (hot shoe) bracket.

Faster shutter speeds are often better when there is significant ambient illumination, and flash is used to flash-fill subjects that are backlit without motion blur, or to increase depth of field by using a small aperture.

X-sync is the simplest to explain and implement: the flash is fired at the instant the shutter is fully open. Electronic flash equipment produces a very short flash

Cameras designed for use with flash bulbs generally had one or more of S (slow) sync, M (medium) sync, F (fast) sync, or FP/FPX (flat peak) sync, designed for use with corresponding bulb types. These sync modes close the contacts a few milliseconds before the shutter is open, to give the flashbulb time to reach peak brightness before exposing the film. Class M bulbs reach their peak illumination at around 20-25 milliseconds after ignition, and class F lamps reach their peak at approximately 5 milliseconds. Most standard flash bulbs used M sync. X sync closes the flash contact just as the shutter blades are almost completely open, and M sync closes the flash contact 20 milliseconds before the blades are completely open.

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Wikipedia