Compound light microscopes have a short focal length objective lens which produces a real image which is examined using a longer focal length eyepiece. The ratio of the focal length of the objective and the eyepiece, when mounted in a standard tube length, gives an approximate magnification of the system. Due to their design, compound microscopes have improved resolving power and contrast in comparison to simple microscopes, and can be used to view the structure, shape and motility of a cell and its organisms, which can be as small as 0.1 micrometres.