Flash Sync Speed
The fastest shutter speed at which a focal-plane shutter is fully open, and therefore the fastest speed at which flash can light the whole frame.
Measured in
seconds
In depth
Above this speed the second curtain starts closing before the first has finished opening, so only a moving slit is ever exposed and the flash lights a band rather than the frame. Typical values are 1/200 s or 1/250 s. Because it caps how much daylight can be excluded, sync speed is the central constraint in mixing flash with sun.
Examples in practice
At the 1/250 s sync limit and f/16 at ISO 100, a flash can just balance bright midday sun.