A shutter controls the length of time for which a film or sensor is exposed to light. Exposure is determined by the brightness of an image and the duration of an exposure. Most single lens reflex cameras, whether film of digital, use a focal plane shutter to control the amount of light falling on the film or sensor. A metal or fabric blind moves across the focal plane to produce a slit through which light passes for the correct exposure period. The exposure period is controlled by the width of the slit and the speed of movement of the blinds. When the shutter mechanism is released, the leading blind retreats to open the slit through which light is admitted. After an interval determined by the shutter speed setting, the trailing blind is released to close the slit and prevent further light from reaching the film or sensor.
One problem with focal-plane shutters is that the shutter blinds take a finite amount of time to traverse the film or sensor plane. One side of the film or sensor is therefore exposed before the other. However, the trailing blind exhibits the same characteristics and shuts off light from one side of the frame before the other. The two effects cancel each other out.
When very short exposures are required, the time taken for the leading blind to traverse the plane of the film or sensor is greater than the exposure period, so the trailing must be released to close the slot before the leading blind has fully opened. This produces a narrow slot which traverses the film or sensor in a progressive manner at a speed which results in the correct exposure. Nevertheless, the leading side of the frame is exposed fractionally before the trailing side.
This produces problems when using electronic flash, because the duration of the flash is very short and the light source may be switched off before the slot between the blinds has traversed the frame. Only a small potion of the film or sensor is then exposed correctly. With focal plane shutters, flash synchronization is therefore only possible at slower speeds where the slot between the blinds is sufficient to allow the complete film plane to be uncovered for the duration of the synchronized flash.
Focal plane shutters can also create distortion when a subject is moving rapidly across the field of view. The subject can move significantly in the time taken for the blinds to cross the frame. The leading edge of the frame therefore records the subject before the trailing edge of the frame is exposed. The wheels of a racing car may therefore become elongated.