|1940 Goodspeed add-on flash synchronizer;|
electrical contacts in shutter plunger
image by Voxphoto (Image rights)
Before flash sync, cameras had to be set to bulb mode (B) for flash photos. In dim light, the photographer would hold the shutter open, perhaps using a squeeze bulb. Then the flash would be manually fired (or in the earliest days, some form of flash powder ignited). The photographer would then allow the shutter to close and the lights could be turned back on.
In the 1940s, add-on flash synchronizers became common. To make a flashbulb shots using a press camera, the photographer would press a button on the flash handle. This would send current both to fire the flashbulb, and to energize a solenoid "tripper" connected to the shutter release. Other styles of synchronizers used the mechanical pressing of the shutter release to close an electrical contact.
Eventually, the manufacturers of shutters began including internal sync contacts; and the ungainly add-on synchronizers vanished from the scene.
Types of Flash Sync
The synchronisation mechanism of the shutter must take into account the delay between initiating the flash and the actual light output; a characteristic of the type of flash equipment. Flash bulbs, which produce their light by burning a small quantity of metal wire, have a delay that is significant relative to typical shutter times. Flash bulbs are coded with a letter representing their synchronisation delay. Many cameras have a switch allowing more than one delay to be selected. Some shutters use the same letter codes for this. Others (for example the Zenit C and the Taylor-Hobson Talykron shutter) allow the actual delay to be set, in milliseconds.
X sync causes a Xenon tube or other type of electronic flash to fire in synchronization with the full opening of the shutter, without any time delay between the two events. Cameras offering X Sync alone can only be used with older M Sync and F Sync flashbulb units at shutter speeds slow enough to allow the slower flash systems to reach their maximum light output before the shutter closes. When speaking about focal-plane shutters, 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. Class M flash bulbs have a firing delay of 18 to 22 milliseconds, so M-Sync is timed to fire the flash 20 milliseconds before the shutter reaches its peak opening, allowing the two events to coincide. 
Cameras that support F Sync or Fast Sync are designed for fast-speed flash bulbs that have a firing delay of approximately 5 milliseconds. A camera with F Sync will fire the flash 5 milliseconds before the shutter reaches its peak opening.
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 finished 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 sync socket|
image by Voxphoto (Image rights)
The standard PC terminal (from "Prontor–Compur") was by 1960 the nearly universal method for connecting a flash unit to a camera. Electrically, a switch inside the camera closes a circuit between the two conductors of the PC connector, just as the shutter opens. This triggers the flash to fire.
A hot shoe is an accessory holder (or accessory shoe) that has electrical flash sync contacts. The center contact and conductive metal sides work exactly as the PC connector does.
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.
|ASA terminal on a 1952 Kodak|
image by Geoff Harrisson (Image rights)
Before the emergence of the PC connector as a global standard, several other flash sync terminals were in use: the ASA bayonet, dual pins, etc.
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 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
Partially to address the low maximum shutter speeds usable with flash, later focal plane shutters, beginning with the Copal Square shutter from the 1960's, first commercially fielded on the Nikkorex F, have lightweight metal or polymer blades, moving vertically. As these moved faster and over a shorter distance (the height of a 35mm frame as opposed to the width,) these allowed synchronization speeds of up to 1/250 sec.