A camera shutter controls both the exact instant when the film is exposed to light and the duration of that exposure. The shutter is used in conjunction with the diaphragm to control the exposure of the film. The most important function of the shutter is that it limits the time that light is allowed to pass through the lens and act on the film. There are two types of camera shutters: leaf and focal plane.

Leaf Shutter

The blades of this type of shutter are usually located between or near the lens elements and close to the diaphragm. It is sometimes called a between-the-lens shutter; however, a more correct term for this type of shutter is a leaf or diaphragm shutter.

Leaf shutters have several blades made of thin spring steel. When the shutter is closed, these blades, or leaves, are at rest and overlap each other. This prevents light from reaching the film. When the shutter release button is pressed, the blades move apart or open quickly

and allow light to pass and expose the film. They remain open for the duration of the preset exposure time before springing shut again (fig. 4-15).

Figure 4-15. Leaf shutter operation.

Leaf shutters have an important advantage over focal-plane shutters. Leaf shutters can be used with electronic flash at all shutter speeds. This is not true with focal-plane shutters. Focal-plane shutters can only be used at slow shutter speeds, usually at 1/125 second and below.

Focal-Plane Shutter

A focal-plane shutter is essentially two lightproof cloths or thin metal curtains that move across the film aperture in the same direction. This type of shutter is housed entirely within the camera body and is mounted on two rollers, one on each side of the film aperture

Figure 4-16. Focal-plane shutter.

(fig. 4-16). As the curtain is moved from one roller to the other by spring tension, the second curtain follows, forming an opening that permits light to pass from the lens to the film. After the opening has passed, the second curtain stops and prevents additional light from reaching the film. In the design of focal-plane shutters, the curtains form a slit that travels across the film aperture to expose the film. When a slow shutter speed is set, the second curtain waits a relatively long time before it follows the first curtain; in this case, the slit is quite wide. When a fast shutter speed is set, the second curtain quickly follows the first and only a narrow slit is formed.