The "rapidity" of a lens depends upon the relation or ratio of the aperture to the equivalent focus. To ascertain this, divide the "equivalent focus" by the diameter of the actual "working aperture" of the lens in question; and note down the quotient as the denominator with 1, or unity, for the numerator. Thus to find the ratio of a lens of 2 inches diameter and 6 inches focus, divide the focus by the aperture, or 6 divided by 2 equals 3; i.e., is the intensity ratio.