Optics.
the ratio of the diameter of a lens, esp. a camera lens, to the focal length; the reciprocal of the f number or focal ratio of the lens.
Optics.
the ratio of the diameter of a lens, esp. a camera lens, to the focal length; the reciprocal of the f number or focal ratio of the lens.
Random House Webster's Unabridged English dictionary. Полный английский словарь Вебстер - Random House . 2012