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f-number, f number
Photog the numerical value of the relative aperture. If the relative aperture is f8, 8 is the f-number and indicates that the focal length of the lens is 8 times the size of the lens aperture
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
f-stop(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.
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