Flash Synchronization - M, F, FP, X and HSS Sync

M, F, FP, X and HSS Sync

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 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 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.

FP sync was used with FP (flat-peak) flash bulbs designed specifically for use with focal-plane shutters. In these shutters, although each part of the film is exposed for the rated exposure time, the film is exposed by a slit which moves across the film in a time (the "X-sync speed") of the order of 1/100"; although the exposure of each part of the film may be 1/2000", the last part of the film is exposed later by the X-sync time than the first part, and a brief flash will illuminate only a strip of film. FP bulbs burned close to full brightness for the full X-sync time, exposing the whole frame even at high shutter speeds.

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, only allow normal 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, although some modern cameras may have an X-sync speed as high as 1/500 (e.g. Nikon's D40 DSLRs). Special electronic flash units for focal-plane shutters fire several times as the slit moves across the film. 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 over a time interval as short as 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.