Flash synchronisation

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.

• At the beginning of the exposure, the cover in front of the image sensor opens from bottom to top, like a curtain in a theatre.
• As soon as this "front curtain" is fully raised, the flash is fired.
• Then, the rest of the exposure time elapses.
• At the end, the cover in front of the image sensor closes again − the ”rear curtain“. However, the curtain does not come down again like in a theatre curtain, because then the lower part of the image sensor would get light longer than the upper part. A second moving part in the camera shutter moves from the bottom to the top.

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:

• Set your camera to camera mode S resp. Tv and switch on the flash, i.e. pop it up if necessary.
• See if you can set a short exposure time of, say, ¹⁄₁₀₀₀ s, or if the camera won't allow it with the flash on.

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

• allows flashes with exposure times as short as you like
• but significantly reduces the flash range for short exposure times.

To switch on short-time synchronisation, you will have to find a corresponding entry in the camera menus.

The technical background is in summary:

• With very short exposure times, there is no point in time when the shutter releases the entire image sensor. The front curtain described above needs a certain time of e.g. ¹⁄₂₅₀ s to rise and if the exposure time is to be shorter, the rear curtain starts to close before the front one is all the way up.
• The flash sends off several bursts of light very quickly in succession, each illuminating a strip of the image sensor. It's an amazing precision that the cameras bring to this.