Number which characterizes the luminous sensitivity of a lens. Another term for aperture value is F-number.
The smaller the number, the more light a lens can collect, the brighter the image, the smaller the depth of field.
The larger the number, the darker the image, but in generally the greater depth of field. At the same time, we generally lose resolution, see Rayleigh Criterion.
The f-number is the ratio of the focal length divided by the apparent size of the aperture (= entrance pupil diameter).
An f = 50mm lens
with an F-number
of F2.0 has to have least one front lens
The inverse of the square of the f-number is a measure for the image brightness of a lens.
The image through a F4.0 lens
only has a quarter of the brightness of an F2.0 lens
four times smaller than
The image through a F5.6 lens
has about twice the brightness of an image through an F8 lens