When the film strip is exposed behind a rotating disk with an open aperture, the camera shutter only allows a portion of the film to be exposed during its angular movement. The wider the shutter opens and stays open, the more light is allowed to pass through to expose the film frame (up to a point, of course). So the shutter speed is based on the number of degrees the shutter rotates from open to closed again.
At 180 degrees, the shutter angle equals the duration of the frame exposure, allowing half the time for the film to be uncovered and half for it to be covered. Any angle smaller than 180 degrees means the shutter blade covers the film strip longer than it uncovers it, resulting in less exposure and a darker exposure. And any angle longer than 180 degrees means the blade uncovers the film strip longer than it covers it, resulting in more exposure and a brighter exposure.
Most movies are shot at a shutter angle of 180 degrees, also called a "180-degree shutter". This is because 180 degrees is the angle that gives the most motion blur. Motion blur is important for creating a smooth and natural-looking image. When the shutter is open for a longer period, the camera will capture more of the motion that occurs between frames. This results in a more fluid and dynamic image.
Here are a few general guidelines for shutter angles:
- Normal Shutter Angle: 180 degrees for a film look and to minimize flicker and "strobing" motion.
- Slow Motion: 360 degrees to reduce motion blur, resulting in the motion appearing much slower.
- Fast Action: 45 degrees or less to blur fast action and eliminate strobe effects.
- Action Films: 90-165 degrees for dramatic camera pans, and high-speed action.
- Creative Shutter Angle: 270-359 degrees for abstract images, such as in old cowboy movies.
If the shutter speed is too slow, it will produce a blurry image. If the shutter speed is too fast, it will produce an image with frozen motion.
Cinematographers use shutter angle to create a desired effect in their images. They take into account the shutter speed, the frame rate, and the motion of the subject when choosing a shutter angle.