The 3pc auto focus extension tube come in different length:
(Canon mount: 13mm, 21mm and 31mm)
(Nikon mount:. 12mm, 20mm and 36mm)
It is made of metal ring, much stronger and more durable. This extension can maintain auto focus, TTL metering and auto exposure. It is much easier to control your camera in low light situation.
Extension tubes are designed to enable a lens to focus closer than its normal minimum focusing distance. Getting closer has the effect of magnifying your subject. They are exceptionally useful for macro photography, enabling you to convert almost any lens into a macro lens at a fraction of the cost.
The extension tubes have no optics. They are mounted in between the camera body and lens to create more distance between the lens and sensor. 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 Automatic Extension Tube Set comes as a set of 3 tubes of different lengths. Use these by inserting between the lens and the body of DSLR Cameras. These set of tubes also can be used singly or combined in seven magnification strength.