There are two shutters or curtains in your camera. One opens to reveal the sensor as you create an image the second one covers the sensor at the end of the shot. You can choose to set you flash to synchronise with either the first curtain opening or the second curtain closing.
When you capture a moving image using ambient light at a slow shutter speeds the image will appear to blur. Capture the same image using just flash and the image would be frozen as the exposure time is the time is the duration of the flash.
If you are using BOTH ambient light and flash you will get a blurred image from the ambient light with a sharp/frozen image superimposed over it from the flash.
If you use first curtain sync the frozen image will be at the start of the motion with the blurring being in front of the subject. This gives the impression of the subject moving backwards. In second curtain sync the frozen image is taken at the end of the exposure and the blurs appear to be behind the subject, thus giving the impression that the subject is moving forward.
When using second curtain-sync you get TWO flashes from your speedlight. The first flash occurs PRIOR to the shutter opening. This is used by the camera to calculate how much light is required from the flash to illuminate the scene. This enables the camera to accurately control when to start the flash so that it shuts of just as the second curtain closes. The problem with this is that if your subject is not aware of the technique you are using they may think the image has been taken when the preflash goes off and look away from the camera of stop moving, thus ruining the effect.
Set you camera/flash to first curtain sync. Ensure you have a shutter speed of 1/4s. You may need to select a low ISO and/or a small aperture to achieve this.
Take a image of a subject walking across the frame. Notice how the subject is frozen by the flash at the start of the motion and the blur is in front of them.
Set your camera/flash to second curtain sync and retake the image. Note the pre-flash and notice how the blurs appears to follow the subject.
This blog post is an extract from the notes which accompany the “Learn to Use your Flash” workshop. The workshop is next running on Sunday 23rd August.