fStop (aperture) and shutter speed/angle

In real-world photography, together the fStop and shutter speed (or shutter angle for film cameras) determine how much light is exposed to film. However, fStop and shutter speed also determine what is in focus, to a certain extent, but for very different reasons.

The length of time light is allowed to pass through the camera lens to the film is determined by the shutter speed. The higher the speed, the shorter the exposure time, the less light exposed to the film.

The amount of light that is allowed to pass through the camera lens to the film is determined by the camera’s aperture setting (also known as the fStop). The wider the aperture, the more light exposed to the film.

NoteA still camera’s shutter speed performs the same function as a film camera’s shutter angle. The shutter angle is a metal disk that is missing a pie-shaped section. This disk sits between the lens and the film, and rotates at a constant rate. When the missing section is in front of the film, it allows light from the lens to pass through and expose the film.

The larger the angle of the pie-shaped section, the longer the exposure time, and moving objects appear more blurry. For more information, see Shutter Angle.

Shutter speed/angle determines motion blur

Motion blur gives the feeling of motion. Motion blur is determined by the shutter speed. The slower the shutter speed (sometimes deliberately done), the harder motion is to stop. That is, fast motion (such as a moving car) appears motion blurred at slower shutter speeds. At higher speeds, the moving car is ‘stopped’ and therefore in focus.

To set motion blur in Maya, see Motion blur.

For information on mental ray for Maya’s motion blur, see mental ray for Maya motion blur.

Aperture determines Depth of Field (DOF)

Depth of field is the region of sharp focus in a photograph. Depth of field is determined by the camera’s aperture setting. At wide aperture settings (for example, at fStop f/2), the depth of field is shallow, and more of the foreground and background (that brackets the area in sharp focus) is out of focus. At narrow aperture settings (for example, at fStop f/22), the depth of field is deep, and more of the foreground and background is in focus.

To adjust a camera’s fStop to affect the depth of field, see Adjust depth of field.