Extend the range of shutter speeds available in mode M; the slowest shutter speed available when  is selected is 900 s (15 minutes). Extended shutter speeds can be used for pictures of the night sky and other long exposures.
At speeds slower than 1 s, the shutter speed displayed by the camera may differ from the actual exposure time. The actual exposure times at shutter speeds of 15 and 30 seconds, for example, are respectively 16 and 32 seconds. The exposure time will again match the selected shutter speed at speeds of 60 seconds and slower.