High speed sync (HSS)

Feature of modern computerised flash units which raises the effective flash sync speed.

Flash sync speed, the fastest shutter speed which can be used with flash, is normally limited to the fastest shutter speed which still exposes the whole frame to light at the same time.

At higher shutter speeds the second shutter curtain starts to move across the frame before the first curtain has completely opened, so the exposure occurs in a slot which moves across the frame. To enable flash to work in this situation, HSS fires a series of short flashes during the exposure which aim to light the whole frame bit by bit.

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