The focal-plane type shutter found in most SLR cameras consists of two opaque blinds which travel across the film plane when the exposure is made. These two blinds are separated by a slit, the width of which determines how much exposure the film receives (thin slit gives less exposure and vice versa); thus the width of the slit determines the effective shutter speed.

When a flash exposure is made, the slit has to be at least as wide as the frame of film, otherwise one of the blinds will obscure part of the frame. The fastest shutter speed for which the slit is as least as wide as the frame is known as the flash-sync speed, and is usually about 1/125s-1/250s.

For shutter speeds higher than the flash-sync speed, the slit will be too narrow and part of the frame will be obscured by the trailing shutter blind, giving a characteristic dark band over 1/2, 1/4 or 1/8 of the frame. Any shutter speed slower that the sync speed will work properly with flash exposures. Using a shutter speed slow enough to record low levels of ambient light in combination with flash is a technique known as slow-sync flash. This technique works well with moving subjects in light requiring, say, a shutter speed of 1/15s - 1/2s, where the subject will be recorded as a moving streak, with a superimposed sharp image frozen by the flash burst.