Flash synchronisation is the coordination of the very short flash pulse with the much longer exposure time. There are more subtleties than one might suspect at first glance.
Flash sync is the coordination of the exposure time with the firing of the flash. The flash fires for only a fraction of the exposure time while the aperture is open and allows light to fall on the sensor.
If we take out some technical terms the subject of flash synchronisation becomes a lot easier.
This is the standard setting usually called simply fill flash: The flash lights up briefly at the beginning of the much longer exposure time – more technically it is synced with the shutter's ”front curtain“.
The expression comes from the type of shutter that was common in SLR cameras decades before digital photography and is still found in digital SLR and mirrorless system cameras today.
You can see this kind of synchronisation when there is motion blur in the picture. The small remote-controlled car drove forwards for this photo. The short flash brought it into focus, then it continued to drive during the exposure time, creating motion blur to the right.
The flash briefly lights up at the end of the much longer exposure time − this is what is meant by sync to ”rear curtain“.
It results in a more natural look when shooting with motion blur, with the blur appearing like a shadow behind the sharp image.
If the camera allows a longer exposure time than the usual limit of 1⁄60 or 1⁄30 s, this is called slow synchronisation. The exposure is adjusted to the dark background and will make it appear brighter. Because of the resulting long exposure times, the camera should be on a tripod or at least propped up somewhere.
Flash exposure compensation can make Freddy appear a little less bright, we will come back to this on our own page on flash exposure compensation.
It may well be that your camera automatically chooses to sync to the rear curtain when you set a slow sync.
This has another advantage besides the more natural looking motion blur: after the flash goes off, everyone you photograph thinks the shot is done and starts moving again. With sync to rear curtain however, people will stay still until the end of the exposure.
With Nikon and Sony, long exposure sync is labelled SLOW.
High speed flash synchronisation is the coordination of the flash with particularly short exposure times, abbreviated HSS or FP (short-time) at Nikon.
This is relevant only if your camera has a flash sync time limit of, say, 1⁄250 s; you'll find it on SLR and mirrorless system cameras. You can also easily try if this applies to your camera:
If your camera has a limit on the shortest exposure time with flash, the next question is whether it and the flash support high speed synchronisation.
Result of the high speed synchronisation: The camera applies a technical refinement that
To switch on short-time synchronisation, you will have to find a corresponding entry in the camera menus.
The technical background is in summary:
This is a special case that misuses the flash and has nothing to do with the actual exposure: Before the exposure begins, the flash fires a few times so that the pupils of people contract and fewer bright red eyes are visible in the photo. This can be combined with any of the other methods described.
My opinion: Don't use it. The multiple flashes are unpleasant, make people you want to photograph blink and have a real chance of spoiling your photo.