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 also makes a
UNITUBE 12 mm or 25 mm which can be purchased individually.
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.
Please Note; 1. When using extension tubes the lens will not
focus to infinity. The focus range will be greatly limited to a very
close focusing distance. 2. There is light fall off when using any
extension tube, sometimes the equivalent of 3 f-stops of light is lost
when using multiple extension tubes together. This light lost can affect
the camera's ability to auto focus. Manual focusing is recommended
should the lens begin to "hunt" (not lock onto the subject). 3. Nikon "D" information is not transmitted because the lens is focusing closer than its programmed to focus. 4.
Kenko Auto Extension Tubes are not auto focus compatible with Nikon
AF-s (Silent Wave) lenses, manual focusing is required with these
lenses.