The JJC Auto focus extension tube set for Nikon F Mount contains three tubes of different length - 12mm, 20mm, and 36mm - which can be used individually or in any combination to obtain the desired magnification. Actual magnification effect changes with each specific lens.</br> </br> 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.</br> </br> 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 farther 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.</br> </br> Tips: When all the tubes are stacked together, you could find the auto focus does not perform very well, this is a problem in general for any brand extension tube set. The more/longer tubes you add the more you have to close up the aperture in order to get the subject in focus, thus the more light you need. This means your camera may have a hard time auto focusing because of the small depth of field you work with (the closer the subject the less depth of field you have). So compensate for it, use manual focus or set the camera on a tripod with a focusing rail.