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.

Published by paleololigo

Scientist (Paleontology, Geochemistry, Geology); Writer (Speculative and Science Fiction, plus technical and non-technical Science); Mom to great boy on the Autism spectrum; possessor of too many hobbies.

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