You can get great shots by combining flash with a long exposure. Usually all you need to do is set your camera to slow sync mode and start shooting; your subject is sharply lit, but you also get ghostly movement blur and trails of light from moving light sources.
However, there’s a problem; in regular slow sync mode the flash fires at the start of the exposure, and then the shutter stays open to record the ambient light and movement blur. If your subject is clearly heading in a specific direction, the effect looks wrong because the blur is traveling ahead of your subject instead of behind it.
That’s why most cameras have a rear curtain sync flash mode – also called second curtain sync or 'pop and burn' – where the flash fires at the end of the exposure, so that the movement trail is behind your subject.
Why isn’t this the default mode for slow sync flash? Because it’s very difficult to time your shots when the flash is at the end of the exposure! It’s worth the effort, though, because when these shots work, they look terrific.
Front curtain sync VS rear curtain sync
These two shots show the difference between front curtain sync and rear curtain sync. Front curtain sync fires the flash at the start (the front) of the exposure, so that the light trails are in front of the bike; rear curtain sync fires the flash at the end (rear), so that the trails are behind it.
01 Set up your shot
You need to plan your composition carefully. Place your subject in the position you want it to be in when the flash fires, but frame the shot to allow room behind for the light trail as well. Now’s a good time to set the focus – the camera won’t track a moving subject in the dark.
02 Switch to manual
The main light will come from the flash, but you still want some of the ambient background light to register and a long enough exposure to record the light trail. We’re using a one-second exposure at f/5.6 (avoid smaller lens apertures because these will sap the flash power).
03 Manual flash power
We could use TTL auto flash, but in situations like this it’s simpler to experiment with the flash power manually. For this you switch your flash to manual mode, then choose the power setting; 1/1 is full power, 1/2 is half power and so on. You need plenty of power for outdoor shots.
04 Rear curtain sync
Now change your camera’s flash mode to rear curtain sync or second curtain sync. The method for this will depend on your camera – and some models require a hotshoe add-on to enable the function.
05 Try some shots
Now you’ll need a willing subject who’s prepared to drive up and down while you get the timing and the positioning right! Remember that you need to time the shot so your subject is in position at the end of the exposure, not the start, and that can take some practice.
06 Final directions
To get proper control over the timing and the length of the light trail, you’ll need to give your subject precise instructions. Our shots look fast, but the bike was only traveling at 12mph! Get your subject to follow a fixed line through the shot, because this will help with focusing.