Flash synchronization

Flash synchronization
An example of a flash firing too late or the shutter speed being too high to sync properly with a SLR, note the different light levels of the top and bottom of the photo.
Hot shoe flash contact

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) 3.5 mm (1/8") connector[1] (as defined in ISO 519[2]), 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 increase depth of field by using a high aperture.


M, F, FP, X and HSS sync

Cameras designed for use with flash bulbs generally had one or more of M (medium) sync, F (fast) sync, or FP (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 put the main pulse of light at the best time relative to the shutter opening. Class M bulbs reach their peak of illumination at around 20-25 milliseconds, and class F lamps reach their peak at approximately 5 milliseconds.[3] 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.[citation needed] 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.[4]

X (xenon) sync is a mode designed for use with electronic flash.[5] 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's D40 DSLRs). 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 situated within the lens housing, open to expose the entire image at once, and therefore 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 flash synchronization at shorter shutter speeds, therefore called high-speed sync (HSS). 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, similar to FP sync. 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. Wireless flash units with this feature are currently very rare.

Rear-curtain sync

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.

Wireless sync

Some synchronization methods employ optical or radio triggering that require no electrical connection to the camera or main flash unit. This allows the camera to move without the restriction of cables. Optical triggering requires at least one flash electrically connected to the camera. A sensor either built-in or external to a remote slave flash unit will sense the light from the master flash and tell a remote flash to fire. Radio triggering requires a transmitter electrically connected to the camera to trigger a remote receiver connected to a remote flash unit.

One of the problems with optical triggering is that in modern digital camera's the in-built flash releases one or more 'pre-flashes'. Many optical slave units will respond to the pre-flash thus firing the slave flash too early. Sometimes this can be prevented by setting the camera to manual ('M'). However, a good number of camera's will still fire pre-flashes even on a manual setting. This is equally true for the compact camera's as well as the more professional digital SLR camera's. Still, a flash connected to the PC jack on a camera or in the hotshoe, will usually not fire pre-flashes in the 'M' setting and therefore can be used to optically trigger a number of slave flashes.

Many compact camera's however, only have a built in flash and no hotshoe and no connector for an external flash and there is no way to avoid the suppress the pre-flash. In those instances, slave units are used that are able to skip a number of flashes, thus skipping one or more pre-flashes and only firing simultaneously with the main flash firing. Modern flash units have this capacity built in. At the low end e.g. is the Godox 18, a simple flash unit that can be set to skip a max of 3 flashes. A more advanced flash that can set to skip one preflash is the popular 'strobist' flash the Lumopro160. Also some studio flashes can be set to ignore pre-flash.

Rather than selecting a special amount of preflashes to ignore, Some slave units have a learning mode in which firing one flash teaches them on which flash to synchronise.

See also


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