On motion picture cameras film is exposed as a pie-shaped mirror, called a rotary disc shutter, rotates in front of the film gate. As the mirror spins it reflects the image into the ground glass so that it can be viewed by the camera operator part of the time. The other part of the time the mirror allows the light to pass on to the film. On simple cameras this shutter is fixed and usually semi-circular. On more advanced cameras the shape of the shutter can be adjusted. This shutter setting is referred to as the shutter angle.
Adjusting the shutter angle controls the proportion of time that the film is exposed to light during each frame interval. The angle of the shutter forms a proportion to the time that each frame of film is exposed:
The primary reason that cinematographers adjust the shutter angle is to control the amount of motion blur that is recorded on each successive frame of film. A tight shutter angle will constrict motion blur. A wide shutter angle will allow it. A 180° shutter angle is considered normal.
Tight shutters create a stuttering stop motion animation look that has become popular in action and war films. In particular tight shutters are used to capture particles flying through the air, such as dirt from an exploding mortar.
On most film cameras the shutter angle is changed by removing the lens and adjusting the shutter with a special tool. This cannot be done while the camera is operating. Some cameras such as the Arriflex 435ES can modify their shutter angle during the shot. This is sometimes referred to as an electronic shutter. An electronic shutter can, for one purpose, compensate for the exposure change caused by a speed ramp without changing the aperture and affecting depth of field.
Other types of shutter adjustment such as an out-of-phase shutter, and a fluttering shutter are also possible. Normally the film is held steady in the gate whenever it is exposed to light. Special shutter adjustments allow the film to be exposed while the camera mechanism is actually moving the film to the next frame. In the film Saving Private Ryan, cinematographer Janusz Kaminski used such a shutter adjustment to give his film the look of World War II newsreel photography. Previously, this effect could only be achieved by purposefully mis-adjusting the timing belt of the film advance mechanism in the camera. However, several manufacturers now provide accessories to electronically control the phase relationship of the shutter and film advance mechanisms. One such device is the Timing Shift Box available for the above mentioned ARRI 435 series.
The term electronic shutter is often used to describe the electronic process of controlling exposure time on a light sensor. The same term is also used in film cameras to refer to a mechanical rotary shutter which can adjust its shutter angle electronically while shooting.