High-Speed Sync

Standard flash synchronisation requires the camera's mechanical shutter to be fully open when the flash fires. With a focal plane shutter, this limits the flash sync speed to the point at which the second shutter curtain begins to close before the first has fully opened — typically 1/250s or 1/320s on most M43 bodies. At shutter speeds faster than this, the second curtain begins moving before the first has cleared the sensor, creating a band of shadow in the frame where the shutter gap was when the flash fired.

High-speed sync resolves this by changing how the flash fires. Instead of a single burst of light, the flash emits a series of rapid pulses throughout the duration of the exposure. Because the flash is continuously illuminating during the entire sensor scan, even the portion of the sensor covered by the moving shutter curtain gap receives flash illumination. This allows the camera to use any shutter speed up to its maximum with flash, regardless of the curtain position.

The trade-off of HSS is reduced flash output. The rapid pulsing distributes the flash's energy over a longer period, reducing the peak intensity available at any given moment. At 1/2000s, a flash may lose 2 to 3 stops of effective output compared to the same flash at standard sync speed. This means HSS is most effective in situations where the flash is used as fill rather than the primary light source, or when using a powerful flash unit that retains sufficient output even at reduced levels. Li-ion battery flashes handle HSS better than AA-battery units because they can sustain higher energy output during the extended pulse sequence.