^* The f number of a lens, seldom explained, is essentially a measure of how much light can get from the outside world into the inside of a camera. The number is very simply calculated by dividing the focal length of the lens (i.e., the distance from the center of the lens to where it focuses light on the film, or the sensor, in the back of the camera) by the diameter of the opening in the lens. A Brownie 127 lens with a focal length of 65 mm would need to have, in order to have its f number 14, a fixed aperture about 4 mm across.