Rotary Disc Shutter

A rotary disc shutter is a type of shutter. It is notably used in motion picture cameras.

Rotary discs are semicircular mirrors that rotate in front of the film gate, and thus expose the film. As the mirror spins it reflects the image onto 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 onto 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.

So for instance, at 24 fps the "Frame Interval" value is 0.04167 sec. ( = 1 / 24 ). Using an "Exposure Time" of 1/50 sec. we get a "Shutter Angle" value of 173°, very close to 180° (normal motion blur effect).

This is the reason of the rule-of-thumb "Use an Exposure Time twice the value of the Frames-per-second to get normal motion blur".

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.