Shutter angle

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

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.

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:

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 ( = 1 / 24 ). Using an "exposure time" of 1/50 second gives a "shutter angle" value of 173°, very close to 180° (normal motion blur effect).

This is the reason for 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.

...
Wikipedia