The f-number, known as the f-value, is a dimensionless number that indicates the opening of the aperture and is based on the human perception of brightness. This subjectively perceived brightness is according to a logarithmic function.
The f-stop number is calculated from the ratio of the focal length to the aperture width. The normalized f-stop series consists of the f-stops f1, f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22 and 32 and results from the ratio of the focal length of a lens to its effective aperture. Between two f-numbers or f-values, there is always the ratio root 2. The first value is the quotient of root 1, i.e. 1. The second value is that of root 2, i.e. 1.4. The third value is that of root 4, i.e. 2, and so on. A small f-number means a large aperture, a large f-number means a small aperture. Large apertures mean a shallow depth of field, small apertures mean a large depth of field.