Front curtain sync is the default flash sync mode on virtually every camera, in which the flash fires the instant the first shutter curtain finishes opening. The shutter is fully open, the flash burst freezes the subject, and the remaining shutter time records the continuous ambient exposure before the second curtain closes. For short shutter speeds and static subjects, this is invisible. For longer exposures involving motion, it produces a specific and usually undesirable look.
The problem is timing. Because the flash fires at the start of the exposure, any motion blur from the ambient light is recorded after the frozen subject, leaving a trail that extends forward in the direction of travel. A runner photographed with a half-second shutter and front curtain sync appears with a sharp, flashlit body, then a ghosted streak racing ahead of them. Viewers read motion blur as inertia behind a subject, so the result looks backward, like the runner is somehow leaving themselves behind.
Switching to rear curtain sync reverses the order: the flash fires just before the second curtain closes, so the ambient trail records first and the frozen burst lands at the end. The blur then sits behind the subject where the eye expects it. Most photographers leave the camera in front curtain for routine fill flash and switch to rear curtain whenever they are deliberately dragging the shutter with moving subjects.
Front curtain sync is not wrong, only context-dependent. For ordinary fill flash in daylight or for typical event work at 1/60 to 1/200, the burst is so much brighter than the ambient that no trail forms, and the choice between front and rear sync makes no visible difference. The decision matters as soon as shutter speeds extend into the half-second range or longer and the ambient exposure starts to register motion on its own.
One subtle benefit of front curtain is timing predictability. The flash fires at the moment the shutter button feels like it triggers, which makes peak-action capture easier. With rear curtain, the flash fires at the end of the exposure, so the photographer has to anticipate by the length of the shutter time. At 1/30 the difference is negligible; at one or two seconds it is a noticeable lag, and missing a peak gesture is common.
Both sync modes operate within the camera’s flash sync speed limit, the fastest shutter at which the sensor is fully exposed during the flash burst. Above that limit, high-speed sync takes over by pulsing the flash through a moving slit, at which point the front-versus-rear distinction stops mattering because the entire sensor is never fully open at once. In practice, front curtain sync is a sensible default that you only need to override when motion blur is the point of the picture.