X-sync is the maximum shutter speed at which a focal plane shutter is fully open across the entire frame at one time, allowing a flash to expose the whole sensor uniformly. Below the x-sync speed, both shutter curtains uncover the sensor completely with a brief interval before the second curtain begins to close. Above the x-sync speed, the second curtain begins moving before the first finishes, leaving only a traveling slit across the sensor, which a single flash pulse cannot illuminate evenly.
Typical x-sync speeds on modern focal plane shutters fall between 1/160s and 1/250s, with some cameras claiming 1/320s or 1/400s using slightly slower second-curtain timing. Leaf-shutter lenses, used in some medium-format systems and on a few specialty cameras, sync at any speed up to the lens’s maximum, often 1/1000s or 1/1600s, because the shutter blades open and close as a single unit rather than as a curtain sweep. Global-shutter sensors sync at any shutter speed by definition, since the whole sensor reads simultaneously.
Beyond the x-sync limit, high-speed sync, abbreviated HSS or FP mode depending on the manufacturer, allows flash use by firing the flash as a long series of rapid pulses that act as a continuous source while the shutter slit traverses the sensor. This works because each row of the sensor sees the slit briefly and the pulsing flash during that brief window, with the net effect of even exposure across the frame. The cost is dramatically reduced flash power, often three or four stops below normal full output, because the total energy is spread across the entire sweep rather than concentrated in one burst.
X-sync matters most when fill flash is needed in daylight at wide apertures. A 1/200s sync speed at f/2.8 in bright sun pushes the ambient exposure into overexposure, forcing the photographer to either stop down (losing shallow depth of field), add a strong neutral density filter, or switch to HSS. Outdoor portrait photographers working with strobes against bright skies become very familiar with the x-sync limit and the trade-offs involved in crossing it.
Symptoms of exceeding x-sync without HSS include a black bar across the bottom or side of the frame, where the second shutter curtain blocked part of the sensor during the flash. The position of the bar depends on the shutter’s direction of travel and the timing of the flash relative to the curtains. Some cameras quietly disable shutter speeds above sync when a flash is detected; others permit it, leaving the photographer to discover the problem in playback.
Common pitfalls include forgetting that electronic shutter modes often have a much lower effective sync speed because of rolling readout, sometimes as slow as 1/30s or less, and assuming HSS is a free upgrade when in fact it costs significant flash power and rules out some lower-tier speedlights entirely. Knowing the exact x-sync of the camera in use is part of being prepared for any daylight flash work.