Extension tubes are designed 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.
The 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.
The KENKO AUTO EXTENSION TUBE SET contains three tubes of different length, a 12 mm, 20 mm, and 36 mm, which can be used individually or in any combination to obtain the desired magnification.
Kenko's Auto Focus extension tubes are designed with all the circuitry and mechanical coupling to maintain auto focus and TTL auto exposure with most Nikon lenses given there is enough light to activate the cameras AF system properly.