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).

The inverse of the square of the f-number is a measure for the image brightness of a lens.