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