Flash sync speed

The fastest shutter speed at which a flash will light the whole frame.

Usually there are two shutter curtains, one of which opens to expose the film or sensor and a second one which closes to end the exposure. When the shutter speed is very fast the second curtain begins to cover the film/sensor before the first curtain has completely opened, and the exposure is carried out as a slot which traverses the frame.

In these circumstances there is no one moment where the whole frame is open to be lit by the almost instantaneous burst of flash light. If flash is used part of the image will be dark because one of the shutter curtains is part of the way across the frame.

The flash sync speed is the fastest shutter speed that can be used without this happening.

A faster flash sync speed is useful when using fill flash in daylight. Portable flash units can struggle to generate enough power to light shadow areas and balance with daylight. A higher shutter speed and corresponding larger aperture can be used, the larger aperture reducing the flash power needed.

---

---