German sources typically used for the relative aperture.

The simpler half-lens was of adequate quality for the narrow angle of view and small relative aperture.

N is the relative aperture (f-number)

The relative aperture is specified as an f-number, the ratio of the lens focal length to its effective aperture diameter.

Because of the higher crop factor, an image shot at a given relative aperture and angle of view has more depth of field on Four Thirds.

The system of f-numbers for specifying relative apertures evolved in the late nineteenth century, in competition with several other systems of aperture notation.

In photography, exposure value (EV) denotes all combinations of a camera's shutter speed and relative aperture that give the same exposure.

The exposure time ("shutter speed") determines the amount of motion blur, as illustrated by the two images at the right, and the relative aperture determines the depth of field.

In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil.

Canon, Pentax and Leica cameras use 'Av' and 'Tv' to indicate relative aperture and shutter speed as well as to symbolize aperture priority and shutter priority modes.

Extremely small apertures such as f/64 can be used without issues of diffraction on the lenses, of much larger focal length than those used for smaller images (a small relative aperture is still a fairly large hole, reducing diffraction).

The 1961 ASA standard PH2.12-1961 American Standard General-Purpose Photographic Exposure Meters (Photoelectric Type) specifies that "The symbol for relative apertures shall be f/ or f : followed by the effective f-number."