If the position of the back principal point of a lens is known, then by focussing on a distant object and measuring the distance from screen to principal point you find the focal length. If the principal point is not known, focus on a distant object, and then rack out and focus on a near one, measuring the extra extension. Then determine the ratio of object to image, multiply the result by the extra extension previously found, and the result is the focal length. It is convenient to use a finely divided scale as the test object, and then measure the image either with the same scale or another exactly similar. As an example, suppose we find that the extra extension of the camera is 2.4 in., and the object is two and a half times the size of the image; then the focal length must be 2.4 multiplied by 2.5 in., or 6 in.