Flash synchronisation, typically referred to as flash sync, is a means by which a flash head is fired at precisely the moment when the camera's shutter is at its peak opening.
Before flash sync, cameras had to be set to bulb mode (B) in complete darkness. A cable release would be used to hold the shutter open, and the flash would be manually fired. The photographer would then allow the shutter to close and the lights could be turned back on. Bulb mode can also be used to open the shutter in low light for longer than the shutter mechanism would otherwise allow.
Types of Flash Sync
X sync causes the Xenon electronic flash to burst in synchronization with the full opening of the shutter. For some manual cameras, the X sync speed refers to the maximum speed that the camera can synchronize with the flash.
Some older cameras support M sync, which supports flash sync with (now obsolete) medium-speed electric flash bulbs. Because flash bulbs take some time to generate their maximum light output, M-sync is timed to fire the flash slightly earlier, then opening the shutter so as to coincide with that peak output.
Because cameras with focal plane shutters use two curtains, acting in sequence, to determine the overall exposure, there is a limit to the shutter speed you can use with flash that is close to instantaneous, including standard, traditional flash bulbs as well as basic electronic flash. Any such flash source is only workable at or below a shutter speed which sees the whole film frame uncovered at the same time, i.e. when the first curtain has fully opened before the second curtain starts to close. With horizontal blind type shutters this generally limits the electronic flash sync speed to a maximum of 1/60 sec.
This limit applies because, in order to achieve higher speeds from a focal plane shutter, the rear curtain begins to close soon after the front curtain has begun opening, forming a "travelling slit" that moves across the film plane. In this case, standard X-sync flash will not work, yielding only a partially exposed frame determined by the width and position of the slit at the time the flash fires.
Focal plane high-speed sync, or FP-sync, traditionally involved special, slow-burning flashbulbs that maintained nominally constant light output over the time it took the travelling slit to complete its journey across the film frame. With electronic flash such extended light output is not feasible, and advanced flash units have been evolved that provide a strobed output (rapidly repeated flash) as that "slit" moves across the film plane. This allows light from the flash head to expose the entire surface area of the film frame (or, these days, the digital imaging sensor) evenly, though at necessarily reduced flash output.
Types of Flash Sync Connections
PC terminal (from "Prontor–Compur") is the standard method for connecting a flash head to a camera. When the flash sync is triggered, a signal is sent from the camera along a wire to the flash telling it to fire.
A hot shoe is an accessory holder (or accessory shoe) that has electrical flash sync contacts. A small, portable flash that has a contact on its "foot" can be connected to a hot shoe, which will cause the flash to fire when you press the shutter release.
Most focal plane shutters are composed of two curtains: a front and a rear curtain. The front curtain slides open to begin the exposure, and then the rear curtain slides closed in the same direction to end the exposure.
Front Curtain Sync
Generally, the flash sync fires at the instant the front curtain has fully opened. This is called front curtain sync, and it is used where it is desired that the flash freeze motion at the beginning of the exposure. Front curtain sync is adequate for most flash-photography. When making long exposures while also firing a flash, front curtain sync creates an effect where any motion blur - from ambient light - appears ahead of the subject, i.e. it appears to leave the subject behind. In the case of a moving motor vehicle in a night scene, for example, this can create an unrealistic effect since its headlamp and tail-lamp trails will extend out in front of it.
Rear Curtain Sync
Some cameras offer the ability to fire the flash at just before the second curtain closes. This is called rear (or second) curtain sync, and it is used to freeze motion at the end of the exposure. When making long exposures while firing a flash, rear curtain sync creates the effect of motion blur trailing the main subject.
Focal Plane Shutter evolution
To partially address the low maximum shutter speeds usable with flash, later focal plane shutters - commencing with the Copal Square shutter from the 1960s - lightweight metal or polymer blades, moving vertically, have replaced the original blinds, permitting sync. speeds of up to 1/250 sec.