The shutter opens and closes to permit light to enter the camera. Obviously, the longer the shutter is open, the more light is permitted to enter. The classic shutter speeds are 1 second, ½ second, ¼ second, 1/8 second, 1/15 second, 1/30 second, 1/60 second, and 1/125 second. Modern cameras offer even more shutter speeds, with speeds of 1/4000 second!