<P> Most shutters have a flash synchronization switch to trigger a flash, if connected . This was quite a complicated matter with mechanical shutters and flashbulbs which took an appreciable time to reach full brightness, focal - plane shutters making this even more difficult . Special flashbulbs were designed which had a prolonged burn, illuminating the scene for the whole time taken by a focal plane shutter slit to move across the film . These problems were essentially solved for non-focal - plane shutters with the advent of electronic flash units which fire virtually instantaneously and emit a very short flash . </P> <P> When using a focal - plane shutter with a flash, if the shutter is set at its X-sync speed or slower the whole frame will be exposed when the flash fires (otherwise only a band of the film will be exposed). Some electronic flashes can produce a longer pulse compatible with a focal - plane shutter operated at much higher shutter speeds . The focal - plane shutter will still impart focal - plane shutter distortions to a rapidly moving subject . </P> <P> Cinematography uses a rotary disc shutter in movie cameras, a continuously spinning disc which conceals the image with a reflex mirror during the intermittent motion between frame exposure . The disc then spins to an open section that exposes the next frame of film while it is held by the registration pin . </P> <P> A focal - plane shutter is positioned just in front of the film, in the focal plane, and moves an aperture across the film until the full frame has been exposed . Focal - plane shutters are usually implemented as a pair of light - tight cloth, metal, or plastic curtains . For shutter speeds slower than a certain point (known as the X-sync speed of the shutter), which depends on the camera, one curtain of the shutter opens, and the other closes after the correct exposure time . At shutter speeds faster than the X-sync speed, the top curtain of the shutter travels across the focal plane, with the second curtain following behind, effectively moving a slit across the focal plane until each part of the film or sensor has been exposed for the correct time . The effective exposure time can be much shorter than for central shutters, at the cost of some distortion of fast - moving subjects . </P>

Where is the shutter located on a camera