(
Focal-
STOP) The f-stop is the "aperture" opening of a camera lens, which allows light to come in. It also determines how much is in focus in front of and behind the subject (see
depth of field). The f-stop is one of the two primary measurements of a camera lens. The other is the "focal length," which establishes how much of the scene is in view (see
focal length).
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.