For a given size of the subject's image in the focal plane, the same f-number on any focal length lens will give the same depth of field. This is evident from the DOF equation by noting that the ratio "u/f" is constant for constant image size. For example, if the focal length is doubled, the subject distance is also doubled to keep the subject image size the same. This observation contrasts with the common notion that "focal length is twice as important to defocus as f/stop", which applies to a constant subject distance, as opposed to constant image size.