The Nikon PK-3 Auto Extension Tube provides 27.5mm of extension, and can be used with the older Nikon SLR Camera bodies and lenses that have a prong on them for meter coupling longer than 50mm, or with a reversed 50mm lens (using a reverse adapter).
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.
Provides perfect coupling and retains both AI aperture and meter coupling. Does not support AF operation.