High-speed Sync
A flash mode that fires a rapid train of pulses during the whole travel of the shutter slit, allowing flash above the normal sync speed.
Measured in
stops lost
In depth
Because the flash must emit continuously rather than in one burst, its effective power drops dramatically — typically two to three stops, worsening as the shutter speed rises. It is the only way to shoot flash at f/1.8 in sunlight, and its cost in power is why high-speed sync and battery-powered strobes are so often discussed together.
Examples in practice
Shooting a portrait at f/2 and 1/2000 s in full sun requires high-speed sync and roughly four times the flash power.