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.
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.
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.
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