Let’s look at the sequence of events when combining flash with long exposures. When the flash fires during exposure, the normal sequence looks something like this (note there may be some slight variation among different camera brands or flash modes):
1. Press shutter button.
2. Curtain A opens.
3. Flash fires.
4. Frame is open for some period of time (determined by your shutter speed).
5. Curtain B closes the frame, ending the exposure.
This is the normal sequence and doesn't really have a name, except to help differentiate it from our next scenario. With Second Curtain Sync (also sometimes called Slow Sync) the sequence goes like this:
1. Press shutter button.
2. Pre-flash fires so the camera can measure and adjust the intensity.
3. Curtain A opens.
4. Frame is open for some period of time (determined by your shutter speed).
5. Flash fires.
6. Curtain B closes the frame and ends the exposure.
To understand the significance, imagine you are taking a picture at night in which moving lights will be visible during the exposure. During the exposure, the lights move across your frame and you are not panning.
During the first (normal) exposure, the flash illuminates the subject on one side of your frame. The lights then form streaks across the frame right over top of the subject, resulting in a weird looking image.
In the second scenario, the shutter opens and the lights record their streaks. Then the flash illuminates the subject. The streaks from the lights lead up to the subject, giving the illusion of speed and creating a generally cool effect.
Depending on the speed and direction of movement and the intensity of the lights, it doesn’t always work this way but one of the great things about digital is that you can review your results and try something different if you didn’t get what you were expecting.
It bears noting that you can’t precisely control exactly when the flash fires when using second curtain synch. So it’s possible in our example scenario that the subject could be out of the frame before the flash goes off. Second curtain synch requires some practice, a bit of finesse and sometimes luck to use well.
