The shutter times precisely how much light reaches the sensor or film. Most camerasthe aperture must be stopped down by one f/stop to f/16. Changing the shutter to 1/250 sec halves the light reaching the sensor, so to achieve the correct exposure, the aperture must be opened up by one stop to f/8. Maintaining this reciprocal relationship between the aperture and shutter speed balanced means keeping the correct exposure but with different effects.