Rear-curtain sync, also called second-curtain sync, is a flash synchronization mode in which the flash fires at the end of the exposure, just before the second shutter curtain begins to close, instead of at the beginning. In flash photography, it is used with slow shutter speeds so that the ambient-light blur of a moving subject appears behind the sharp, flash-frozen image, which reads as natural forward motion.
The mode matters only when the exposure is long enough for ambient light to record visible movement. At short shutter speeds with little ambient light, rear-curtain and front-curtain sync produce practically identical images.
How rear-curtain sync times the flash
A focal-plane shutter uses two curtains. The first curtain opens to start the exposure and the second curtain closes to end it. At shutter speeds up to the flash sync speed, typically between 1/160 and 1/250 second, there is a moment when the whole sensor is uncovered, and the flash must fire within that window.
In the default front-curtain sync, the flash fires as soon as the first curtain is fully open. In rear-curtain sync, the camera waits until the end of the exposure and fires just before the second curtain starts moving. The flash pulse itself lasts only around 1/1000 to 1/20,000 second, so it freezes the subject at whatever position it has reached at that instant.
| Moment | Shutter state | What is recorded |
|---|---|---|
| Exposure start | First curtain opens | Ambient light begins to accumulate |
| During exposure | Sensor fully uncovered | Moving subject records as a blur trail |
| Flash moment | Just before second curtain moves | Sharp image of the subject at its final position |
| Exposure end | Second curtain closes | Recording stops |
Above the sync speed, the second curtain starts closing before the first has fully opened, so only a moving slit of the sensor is exposed at any moment. A single flash pulse would then light only a strip of the frame, which is why rear-curtain sync is limited to speeds at or below the sync speed and is most useful at speeds far slower than it.
Rear-curtain sync compared with other sync modes
The choice of sync mode decides where the sharp image sits relative to the blur. The table compares rear-curtain sync with the modes it is most often confused with.
| Sync mode | Flash timing | Effect on moving subjects | Typical use |
|---|---|---|---|
| Front-curtain sync | Start of exposure | Blur appears ahead of the subject | Default for most flash work |
| Rear-curtain sync | End of exposure | Blur trails behind the subject | Motion effects, light trails |
| Slow sync | Start of exposure with long shutter | Brighter background, front-curtain look | Night portraits |
| High-speed sync | Pulsed over the whole exposure | No ghosting, flash power reduced | Fill flash in bright sun |
Many cameras combine slow sync with rear-curtain sync, allowing long shutter speeds in automatic modes. High-speed sync and rear-curtain sync are usually not available together, because high-speed sync spreads the flash across the whole exposure rather than placing it at one moment.
Settings that make rear-curtain sync effective
Rear-curtain sync only shows its effect when the ambient exposure and the flash exposure are both significant. The balance between them decides how strong the motion trail looks compared with the frozen subject.
| Parameter | Typical range | Effect |
|---|---|---|
| Shutter speed | 1/30 to 2 seconds | Longer times create longer trails |
| Ambient exposure | About 1 to 2 stops under | Keeps trails visible but not dominant |
| Flash exposure compensation | -1 to 0 EV | Controls how bright the frozen subject is |
| ISO | Low to moderate | Avoids overexposed trails in bright scenes |
| Camera movement | Tripod or deliberate panning | Static or dynamic backgrounds |
Panning with a moving subject while using rear-curtain sync produces a sharp subject on a streaked background, a classic technique for cyclists, dancers and cars at dusk.
Pitfalls of rear-curtain sync
The most common difficulty is timing. Because the flash fires at the end, the subject must be framed where it will be when the exposure ends, not where it was at the start. With exposures of a second or more, people may also move out of position or close their eyes before the flash fires.
- TTL pre-flash timing: the metering pre-flash fires at the start, the main flash at the end, so subjects may blink or react between them.
- Ghosting on bright subjects: strong ambient light creates a visible second image overlapping the frozen one.
- Wireless trigger limits: some radio and optical systems do not support rear-curtain timing reliably.
- Electronic shutter restrictions: many cameras disable rear-curtain sync or all flash with a fully electronic shutter.
- Leaf shutter behavior: some lens shutters offer rear sync only in specific bodies or modes.
When subjects need precise positioning, many photographers prefer to shorten the exposure and accept a shorter trail rather than risk a missed moment.
Creative applications
Rear-curtain sync is a creative tool for expressing motion. It is used for dancers with flowing fabric, runners and cyclists at night, fire spinners, light painting with a final flash on the subject, and event photography where colored stage lights should leave trails behind moving guests.
In each case the key decision is the ratio of ambient blur to flash sharpness. A test sequence that varies shutter speed by one stop at a time is the quickest way to find the balance.
Rear-curtain sync also works with off-camera flash when the trigger system supports it. Placing the flash to the side of a moving subject adds directional modeling to the frozen image, while the ambient trail keeps the sense of motion. Testing the trigger at slow speeds before an event confirms that the flash really fires at the end rather than the start.
A non-technical view
Rear-curtain sync means the flash goes off at the very end of a longer photo instead of at the start. While the shutter is open, a moving person leaves a soft blur in the picture. The flash then captures them sharply at the last moment.
The result looks natural. The blur appears behind the person, like the streaks drawn behind a running character in a cartoon. That is why photographers use it to make movement look fast and lively.
A concrete example
An event photographer covers a dance floor at a wedding reception, lit by colored stage lights. With front-curtain sync, the light trails from swinging arms appear in front of the dancers, which looks as though they are moving backward.
The photographer switches to rear-curtain sync at 1/8 second, f/4 and ISO 800, with the flash bounced off the ceiling at -0.7 EV and a slight camera twist during exposure. The colored lights now form trails that follow the dancers’ movements, and the flash freezes their faces sharply at the end. The resulting images convey the energy of the party while keeping every guest recognizable.