The burst of light emitted by a flash unit is exceedingly short, ranging from 1/1,000th up to 1/10,000th of a second and even faster. To capture such quick bursts of light, the shutter's operation must be synchronized with the flash so that it's fully open at the exact moment the flash goes off. If it isn't, only part of the image or nothing at all will be captured on film.

Cameras with focal-plane shutters (almost all SLRs are in this category) only synchronize at certain speeds. (Cameras that use leaf shutters, which expose the image in a different way, work with flash at all speeds.) This doesn't mean you can't use shutter speeds other than the sync speed when shooting with flash. The flash will also sync with slower shutter speeds than the sync speed. Using those speeds will let in ambient light in addition to the flash. Higher speeds, however, will be out of sync with the shutter.

If you're using a dedicated flash unit, you don't have to worry about flash sync, as the system will do it for you. Most systems will choose the fastest possible shutter speed to lessen the amount of ambient light hitting the film or photo receptors. You'll have to manually set the shutter speed to override this if you want exposures with ambient light as well. With automatic flash units, just attach the flash to the camera and set the shutter speed dial to the proper sync speed (you'll find it in your camera's manual).