Slow-sync flash is a technique where the freezing effect of the flash is mixed with along ambient light exposure to create an image which is both sharp and blurred at the same time. To darken the background so that the frozen subject stand out more, the shutter speed can be increased over that recommended by the camera’s light meter.
• Set the camera to a low ISO setting
• Select a slow shutter speed to allow movement blur, e.g. 1/8 sec.
• Take an ambient light meter reading and select the correct aperture.
• Set the flash unit to give a full exposure at the selected aperture.
• Pan or jiggle the camera during the long exposure.
Adapted from Digital Photography Essential Skills by Mark Galer (Focal Press, 2008, $30)