F is for f-stop – #AtoZChallenge – 2020

F is for f-stop

The f-stop or f-number (denoted N) is used to represent the size of the aperture in a camera. It’s usually written with the script f followed by the number, e.g. f/1.4. In general, smaller f-numbers mean the aperture is more open which results in more light getting to the film.

The number itself (N) is a ratio between the focal length (f) of the complete camera plus lens and the diameter of the opening of the camera’s aperture (D):

N={frac {f}{D}}

The focal length is the distance between the front of the lens and the focus point on the film.

F’ is approximately where the film would be in a camera. f’ is the focal length – the distance between the lens and the film. CREDIT: JiPaul / from Henrik on Wikimedia Commons CC By-SA 3.0

D is the measurement across the aperture from one side to the other. It changes as we adjust the aperture. This value being on the bottom of the fraction is why larger f-numbers mean smaller apertures.

Typical lenses have these f-number settings:

f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, and f/32

Older lenses tend to have a narrower range and often lack the very low f-numbers because the aperture is too small.

You’ll see these numbers with or without the f/ somewhere on the lens like on this Argus C3:

The aperture adjustment is across the top of the lens. This lens is set on f/8. The phrase “f/3.5 50mm” across the bottom shows the user the lowest f-number possible for the lens as a measure of lens ‘speed’ and provides the focal length for the lens. In this photo, you can also see the open aperture behind the lens.

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