The Zeikos 3-Piece Automatic Extension Tube Set is designed specifically for use with Canon digital SLR cameras and lenses ranging from macro to semi-telephoto and allows the photographer to enable a lens to focus closer than its normal set minimum focusing distance. Getting closer has the effect of magnifying your subject (making it appear larger in the viewfinder and in your pictures). They are exceptionally useful for macro photography enabling you to convert almost any lens into a macro lens at a fraction of the cost while maintaining its original optical quality. These extension tubes have no optics. They are mounted in between the camera body and lens to create more distance between the lens and film plane. By moving the lens father away from the film or CCD sensor in the camera the lens is forced to focus much closer than normal. The greater the length of the extension tube the closer the lens can focus. This set contains three tubes of different length -- 13mm 21mm and 31 mm -- which can be used individually or in any combination to obtain the desired magnification. These extension tubes are designed with all the circuitry and mechanical coupling to maintain auto focus and TTL auto exposure with most Canon lenses given there is enough light to activate the cameras AF system properly.