Camera shutters are mechanisms that allow a certain amount of light to pass through the camera in order to expose the film. Film is light-sensitive and too much exposure can wash out the lighter colors in the image. Too little light, on the other hand, can cause a loss of definition in the darker parts of the image. The shutter helps the photographer ensure that the film is exposed to light for the right amount of time. There are several types of camera shutters for different cameras and purposes, though most fit under the umbrella of either between-the-lens or focal-plane shutters.
Between-the-lens shutters, also called lens-plane shutters or central shutters, operate between the elements of the lens, or the materials that make up the lens. These camera shutters are composed of a round metal casing that holds a set of blades in its center. The blades remain closed tightly to block out light until a release is pressed, causing the blades to spring back over the metal casing. This reveals an opening for light to pass through for a designated amount of time.
Focal-plane camera shutters are situated toward the back of the camera directly in front of the film, or the focal plane of the camera. These mechanisms are typically composed of a set of curtains that open to reveal a slot through which light can travel, and then close to block out light. The length of exposure is determined by how far the curtains open. If the first curtain opens fully before the second curtain closes, the exposure is longer. If the first curtain only partially reveals the slot before the second curtain closes, the exposure will be longer.
In more simple designs, there is only one curtain with several different sized slots cut out. The curtain passes over the focal plane, exposing the film for the length of time it takes the width of the slot to pass. Narrower slots allow for shorter exposure, while wider slots give a longer exposure. The curtains are usually made of cloth, though some cameras may have metal focal-plane shutters.
Because the focal-plane shutter is not placed within the lens, it is often used in cameras that take interchangeable lenses. The length of exposure can also be more controlled and much faster with focal-plane camera shutters than with central shutters, allowing for high speed photography. Central shutters are generally more able to synchronize exposure time with the use of flash than focal-plane shutters. Central shutters are also simpler and cheaper to construct, so they are often available on less expensive cameras.
The two lenses may be combined in the same camera to avoid problems with flash photography or fast moving subjects, which may become distorted when using only a focal-plane shutter. Specialized focal-plane camera shutters have also been designed to address these problems. Electro-optical shutters are sometimes used to sidestep issues with flash synchronization and fast moving subjects. A revolving drum shutter can be used to get panoramic pictures without distortion.