-The shutter on a camera controls the exposure of the film to light. 
-The Nikon N55 has a traveling slit shutterr which is used in camera with interchangeable lenses. 
-The film is covered by a thin sheet of mettal or fabric that is pulled away by a spring to initiate exposure. 
-To end the exposure, a second sheet is pullled over the film from the opposite side so that the two sheets chase one another across the film.  -The two sheets are moved by springs that wind when the film is advanced.