A camera's shutter must be fully open at the moment a flash fires if an image is to be correctly exposed. Compact cameras with fixed lenses generally have leaf shutters, built into the lens, which open like an expanding circle to expose the film or sensor. Shutters of this type are therefore always fully open during an exposure, and a flashgun can consequently be fired during the period of the exposure without difficulty.
Single-lens reflex cameras with detachable lenses use larger focal plane shutters located directly in front of the film or sensor. These use a pair of moving curtains, or sliding metal leaves, to cover and uncover the frame during the exposure period. The blades of a focal-plane shutter move at a limited speed and as the shutter speed becomes faster, and the period of exposure decreases, a point is reached where the trailing curtain must begin closing before the leading curtain has fully opened. The result is a slit opening which moves across the frame to progressively expose the film or sensor. Above this so-called flash synchronization speed there is therefore no moment when the whole frame is completely exposed. Under such circumstances, a high-speed burst of light from a flash can expose only part of the sensitized medium.
The flash synchronization speed of a shutter therefore represents the highest speed at which the whole frame can be simultaneously exposed. A typical flash synchronization speed for a modern focal-plane shutter is 1/250 sec or 1/300 sec, although photographers should check the specification of their particular camera.