Focal-plane Shutter
A shutter made of two curtains that travel across the sensor just in front of it, exposing the frame as a moving slit at high speeds.
Measured in
seconds
In depth
Because the shutter sits in the body rather than in each lens, every lens can be simple and cheap. Above the sync speed the second curtain begins closing before the first has finished opening, so only a travelling slit ever exposes the frame — which is why flash cannot illuminate the whole image beyond that threshold, and why fast-moving subjects can shear.
Examples in practice
At 1/8000 s the slit crossing the sensor may be under a millimetre wide, even though the curtains take about 1/250 s to travel.