(Focal-STOP) The measurement of the aperture setting in a camera lens. The f-stop setting determines how much light is allowed to enter the lens and pass through to the film (analog) or CCD or CMOS sensor (digital). It also determines how much in front of and behind the subject is in focus (see depth of field).

Length Divided by Opening
The f-stop is the focal length of the lens divided by the diameter of its opening. Each consecutive f-stop halves the opening of the previous. For example, an 80mm lens with its f-stop set to f8 means that the optics inside the lens create a diameter equivalent to 10mm. Changing the f-stop to f16 creates a diameter of 5mm. See focal length.