Shutter is a device that allows light to pass for a determined period of time, for the purpose of exposing photographic film or a light-sensitive electronic sensor to light to capture a permanent image of a scene. The amount of light that reaches the sensor or the film plane depends upon how long the shutter is open. The slower the shutter speed, the more light enters, while faster the shutter speed, lesser the light entering. shutter is measured in time. A slow shutter speed is <1/30th of a sec while a fast shutter speed is >1/125th of a sec.