Autofocus systems use a special light measuring device to determine subject distance. The device is housed in an AF module located in the interior of the camera. Light passing through the lens passes through the main mirror to a sub-mirror and then into the AF module. This module has a lens system with a fixed aperture of f/7. AF-compatible lenses with apertures larger than f/6.7 can be successfully used with the Dynax AF system. If a lens with an aperture smaller than f/6.7 is used, the AF module cannot "see" to focus. The greatest benefit of a converter comes from using it with a telephoto lens. Most telephoto focal lengths in the Dynax system are offered in zoom lenses. Dynax zoom lenses have a maximum aperture of f/3.5 to f/4.5 or f/4.5 to f/5.6. If a 2x converter were used with one of these lenses, the maximum aperture would be converted to f/6.7 to f/9 or f/9 to f/11 respectively. In the first case, the AF system would work at the lens’ widest focal length but stop working as soon as it is zoomed out. In the second case, it would not work at all. Both lenses could be manually focused but the darker viewfinder caused by the smaller apertures would make this difficult.