After the film is exposed to light coming through the lens, the shutter must close to prevent the light from hitting the film. The film must be completely at rest before the shutter opens again for the next exposure. If the shutter does not block the light from the film when it is moving, the image on the film will be blurred. The simplest kind of shutter is a rotating disc with a section removed.
Exposure is determined by the intensity of the light that passes through the lens and the time or duration of exposure.
Standard film speed is 24 frames per second (fps). A camera with a 180-degree shutter admits light to the film half the time (the disc is half open) so the exposure time (the shutter speed) is 1/24 x 1/2 = 1/48 second (rounded off to 1/50 of a second).
For shutter openings less than 180 degrees, the shutter speed is faster than 1/50 of a second. For example, a 135 degree shutter at 24 fps yields a shutter speed of 1/24 x 135/360 = 1/64 (approximately 1/65 of a second).
In general, the longer the time of exposure, the better. The longer the time of exposure, the less light needed for proper exposure, Shutter speeds shorter than 1/60 of a second under flourescent light may result in a pulsing or flickering of the image.
On cameras equipped with a variable shutter, the shutter angle can be narrowed to change shutter speed. Some variable shutters may be shut down continuously, while others can only be shut down to certain angles.
Narrowing the angle reduces shutter speed. A 90 degree shutter, for example, gives a shutter speed of about 1/100 at 24 fps (using the above formula for shutter speed). Closing the shutter reduces the exposure, allowing high-speed film to be used outdoors or allowing the lens to be opened to decrease depth of field.
A variable shutter that can be closed down while the camera is running allows exposure changes in the middle of a shot. For example, when the camera moves from a sunlit to a shaded area within a shot, it is often necessary to change exposure. The iris diaphragm of the lens can be changed, but this would change the depth of field and may be more noticeable than shutting down the variable shutter. If the variable shutter can be shut down continuously to 0 degrees with the camera running, in-camera fades and dissolves can be created.
Light may be diverted from the lens to the viewfinder screen by a mirror shutter. The mirror, either part of the shutter itself or rotating in synchronization with it, alternately allows all the light to hit the film, and then, when the shutter is closed, all the light to go the the viewfinder. When the camera is stopped, all the light is always available for the viewfinder; when it is running, the light goes to the viewfinder only half the time. During filming, the viewfinder image is thus only half as bright. One paradox of the mirror shutter is that you see an image in the viewfinder during nonexposure, but the viewfinder image blackens during exposure.
Film in a motion picture camera receives exposure through intermittent action. The shutter blocks the light from the film when it is moving. The simplest kind of shutter is a rotating disc with a section removed. The Variable Shutter and the Mirror Shutter are two of the main types of shutters.