Rotary
shutters are semicircular discs that spin in front of the
film gate, alternately allowing light from the lens to strike the film, or blocking it. When blocking the light, the exposed portion of the film is advanced, and another, pristine frame is moved into position. The spinning disc then exposes the new frame of film. The nature of light-sensitive film requires a shutter to block the light and prevent
smearing of the image as the film is advanced through the camera. Digital sensors do not require a rotary shutter, although some high-end
digital cinema cameras do have them. Some rotary shutters utilize mirrors (or mirrored surfaces) so that when the shutter is in its
closed position, light from the lens is redirected to a viewing system, allowing the
camera operator to view, frame, and focus the image, exactly as the film sees it.
Shutter Angle On more advanced cameras, the open portion 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: : \frac{\text{shutter angle}}{360^\circ} = \frac{\text{exposure time}}{\text{frame interval}} . 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. So for instance, at 24
fps the
frame interval value is 0.04167 second ( = ). Using an
exposure time of second gives a shutter angle value of 173°, very close to 180° (normal motion blur effect). 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 compensate for the exposure change caused by a
speed ramp without changing the aperture and affecting
depth of field. Other types of shutter adjustments, 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 Kamiński 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. ==Electronic equivalent==