The figures of the following lens stops, f/4, f/5.6, f/8, f/11.3, f/16, f/22, f/32, f/45, and //64, represent fractions of the focal length, or, roughly, the proportion which the diameter of the stop bears to the distance between the stop and the ground glass when a distant object is focussed. To state the diameter, therefore, it is necessary to know the focus of the lens. Focus an object its exact size, measure the distance between the object and the ground glass image and divide by four. This gives the equivalent focus, and avoids the necessity of measuring from the optical centre. If one is substituted for f, calling it 3/4 in., etc., it is merely necessary to draw a line equal to the focus and divide it into this number of equal parts to obtain the diameter. This is not scientifically accurate, as there is a slight condensation of light by the front lens, but it is near enough for all practical purposes.