Coordinating Your Digital Camera’s Flash and Shutter
Digital cameras use shutters to expose the sensor when you take a picture. Because the flash is a short burst, the flash must be fired only when the shutters entirely uncover the sensor. You can synchronize the flash with the shutters in several ways, each producing a slightly different effect.
Using front-curtain sync.
This is the cameras’ default mode. The electronic flash is triggered as soon as the front curtain opens completely. This records a flash exposure of the subject. Then, the shutter is allowed to remain open for the rest of the exposure. If there’s sufficient ambient light and the subject is moving, you end up with a streak in the direction of the subject movement, as if your subject was preceded by a ghost (see this figure).
Using rear-curtain sync.
In this mode, the flash doesn’t fire until the rear curtain begins to move at the end of the exposure. The ghost image, if any, registers first and terminates with a sharp image at your subject’s end position. In this case, the ghost image trails the subject, as you can see in this figure.
Setting the shutter speed high enough that the ambient light doesn’t register an image.
Use this setting if you don’t want any streak at all in your image (like in this figure). The higher your camera’s flash sync speed, the better your opportunity for using a shutter speed that’s so brief you won’t get a ghost image.
Don’t confuse front- and rear-curtain sync with slow-sync, which is a digital SLR camera option in automated modes that sets the camera to choose slow shutter speeds to deliberately add exposures from ambient light to the flash image.