The shutter controls the amount of time that the light can strike the imaging sensor. Along with the aperture that controls the amount of light, the shutter speed works in combination to control the total amount of light the imaging sensor receives. With the same aperture, using a shutter speed of 1/2 sec. decreases the amount of light by half compared to a shutter speed of 1 sec. And a shutter speed of 1/4 sec. yields one-fourth the amount of light. Unlike the aperture, the difference in the shutter speed readily corresponds to the difference in the amount of light. The shutter speed is indicated by the denominator. For example, "500" means 1/500 sec. Since the sensor can record an image only while the shutter is open, it can freeze (with a fast shutter speed) or blur (with a slow shutter speed) a moving subject.