F-number
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An f-number is a measure of the light-gathering ability of an optical system such as a camera lens. It is calculated by dividing the system's focal length by the diameter of the entrance pupil. The f-number is also known as the focal ratio, f-ratio, or f-stop, and it is key in determining the depth of field, diffraction, and exposure of a photograph. The f-number is dimensionless and is usually expressed using a lower-case hooked f with the format f/N, where N is the f-number.
A 35 mm lens set to f/11, as indicated by the white dot above the f-stop scale on the aperture ring. This lens has an aperture range of f/2 to f/22.
Comparison of f/32 (top-left half) and f/5 (bottom-right half)
Shallow focus with a wide open lens
Aperture
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In optics, the aperture of an optical system is a hole or an opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image of the aperture and focal length of an optical system determine the cone angle of a bundle of rays that comes to a focus in the image plane.
Different apertures of a lens
In biology, the pupil (appearing as a black hole) of the eye is its aperture and the iris is its diaphragm. In humans, the pupil can constrict to as small as 2 mm (f/8.3) and dilate to larger than 8 mm (f/2.1) in some individuals.
A camera aperture
Alvin Clark polishes the big Yerkes Observatory Great Refractor objective lens, with 40 inches 102 cm across, in 1896.