Flash sync speed, often called X-sync speed, is the shortest exposure time at which every part of the sensor is uncovered simultaneously, so that one short flash burst can illuminate the whole frame evenly. In flash photography, it is a property of the camera’s shutter, typically between 1/160 s and 1/250 s for focal-plane shutters, and it sets the upper limit for normal flash work.
Exceeding the sync speed causes a dark band across part of the image, because one shutter curtain is already covering the sensor when the flash fires. Understanding why this happens explains high-speed sync, leaf shutters, electronic shutter limits and many outdoor flash decisions.
Why focal-plane shutters limit flash sync speed
A focal-plane shutter exposes the sensor with two curtains that travel across it in sequence. Exposure begins when the leading curtain uncovers the frame and ends when the trailing curtain covers it again. Each curtain needs a fixed travel time, usually around 2.5 to 4 ms in modern cameras, regardless of the selected shutter speed.
At slow shutter speeds, the first curtain finishes opening before the second begins to close, leaving the sensor fully uncovered for a moment. A flash burst, which typically lasts from about 1/1000 s to 1/20000 s, fits into that window. At faster speeds, the second curtain starts closing before the first has finished opening, and the sensor is exposed only through a moving slit. A flash fired then lights only the strip of sensor visible through the slit.
| Shutter speed | Sensor exposure pattern | Result with normal flash |
|---|---|---|
| Slower than sync | Fully open for a period | Even flash exposure |
| At sync speed | Fully open for a brief moment | Even flash exposure |
| Slightly faster | Wide moving slit | Dark band at one edge |
| Much faster | Narrow moving slit | Only a thin lit strip |
Typical sync speeds by shutter type
The sync speed depends on the shutter design, not on the flash unit. Faster-moving curtains allow higher sync speeds, while shutters that sit in the lens or electronic readout behave very differently.
| Shutter type | Typical sync speed | Main limitation |
|---|---|---|
| Mechanical focal-plane | 1/160 to 1/250 s | Curtain travel time |
| Electronic first curtain | Similar to mechanical | Mechanical second curtain |
| Full electronic shutter | About 1/15 to 1/250 s | Sensor readout speed |
| Global shutter sensor | Up to the fastest shutter speed | Flash duration itself |
| Leaf shutter in lens | Up to about 1/2000 s | Blade speed at small sizes |
A full electronic shutter reads the sensor line by line, which is effectively a slow slit, so many cameras restrict flash or disable it entirely in that mode. A global shutter exposes all pixels at once, which removes the conventional sync limit, while a leaf shutter opens from the center outward and exposes the whole frame at every speed.
Flash sync speed and high-speed sync
High-speed sync (HSS, also called FP mode) is the standard workaround for shutter speeds above the sync limit. Instead of one burst, the flash emits a rapid series of pulses that behaves like continuous light for the time the slit crosses the sensor.
| Aspect | Normal flash at sync speed | High-speed sync |
|---|---|---|
| Light emission | One short burst | Rapid pulse train |
| Effective power | Full guide number | Often 1 to 2 stops or more lower |
| Motion freezing | Set by flash duration | Set by shutter speed |
| Recycle and heat | Normal | Higher load on the flash |
| Typical use | Studio, indoor events | Wide apertures in sunlight |
The power cost of HSS is the key trade-off. Because the light is spread over the whole curtain travel and only part of it reaches the sensor at any instant, effective output falls sharply, and the loss grows as shutter speed rises. The flash must be closer or more powerful to achieve the same exposure.
Using flash sync speed in practice
In practice, the sync speed governs how ambient light and flash are balanced. The shutter speed controls ambient exposure, while the flash exposure depends mostly on aperture, flash power and distance. At or below the sync speed, the photographer can lower ambient brightness by shortening the shutter time without affecting the flash.
- Outdoor fill flash often needs the full sync speed to keep a sunny background from overexposing at moderate apertures.
- Indoor events usually work at 1/60 to 1/200 s, well below the limit, to retain some ambient light.
- Off-camera flash through radio triggers can add delay, so the reliable maximum may be a step below the rated value.
- Hypersync or tail sync, offered by some trigger systems, fires a long-duration flash early to use its tail at speeds above sync, with uneven results that must be tested.
- Rear-curtain sync fires the flash at the end of a long exposure and is unrelated to the speed limit itself.
The common outdoor dilemma is that a wide aperture such as f/2 in sunlight needs a shutter speed far above 1/250 s. The options are HSS with its power loss, an ND filter that reduces ambient and flash equally, or a smaller aperture that accepts more depth of field.
Common mistakes and misconceptions
The most common misconception is that sync speed limits motion freezing. With normal flash in a dark setting, the effective exposure time is the flash duration, which at low power can be shorter than 1/10000 s, far shorter than any shutter speed.
- A dark band at the bottom or top of a frame indicates a speed above sync, not a faulty flash.
- Assuming the same sync speed in electronic shutter mode leads to banding or a blocked flash.
- Using HSS indoors without need wastes power and slows recycling.
- Ignoring trigger latency with third-party radio systems can push the true limit lower than the camera specification.
- Expecting full flash power with ND filters to overcome sunlight; the filter cuts flash and ambient by the same amount.
Put simply
Most cameras use two small curtains to make a picture. One opens, and the other closes. At slower settings, there is a moment when the whole sensor is open. The flash fires during that moment and lights the whole picture.
At very fast settings, the two curtains move together like a narrow window sliding across the sensor. A flash then lights only part of the picture. Flash sync speed is the fastest setting that still works with an ordinary flash burst.
In practice
A portrait photographer works outdoors at noon with a camera whose sync speed is 1/250 s. The plan is a backlit portrait at f/2 with an off-camera flash in a small softbox. In this light, the correct ambient exposure at f/2 and ISO 100 needs about 1/4000 s, four stops above the sync limit.
The photographer considers HSS but finds that the speedlight, now losing much of its effective output, cannot match the sun from 1.5 m. Instead, a four-stop ND filter is mounted, which allows 1/250 s at f/2 with the background exposed as metered, and the flash is raised to full power. The final images show a bright face against a rich sky, with shallow depth of field and no banding.