Shutter Speed Ring The length of time that light is allowed to strike the film is controlled by the focal plane shutter. The shutter consists of two opaque "curtains" which travel across the film surface. The coordinated movement of these curtains determine the exposure time for your picture: 1000 indicates 1/1000 of a second and 60 indicates 1/60 of a second. The figure 1 indicates one full second. At B (Bulb) setting the shutter will remain open as long as the shutter release button is held down.