where formula_2 is the focal length, and formula_3 is the diameter of the entrance pupil ("effective aperture"). It is customary to write f-numbers preceded by , which forms a mathematical expression of the entrance pupil diameter in terms of f and . For example, if a lens' focal length were 10 mm and its entrance pupil diameter were 5 mm, the f-number would be 2. This would be expressed as "2" in a lens system. The aperture diameter would be equal to formula_4.