During a macro photography workshop that I recently attended in the Adirondack Mountains, it became evident that extension tubes are a very useful accessory. When placed between the camera and the lens, extension tubes allow the lens to focus closer to the subject.
I borrowed an extension tube to get a closer perspective of a daisy. Here's the original image without tubes:
The use of just one extension tube allowed me to get in much closer. Extension tubes can also be stacked to allow the lens to move in much closer.
I found the extension tubes to be such a powerful tool that I ordered a set once I returned home. Unfortunately, many brands of extension tubes will not work properly with my Nikon 105VR macro lens. Nikon's own brand of extension tubes will not allow one to adjust the aperture of the 105VR! The Nikon, as well as many other brands, will only work properly with lenses that have a manual aperture ring.
However, the Kenko extension tubes do provide the ability to adjust the aperture as well as provide auto-focusing capabilities. Since the tubes are hollow and don't contain any optical elements, I elected to save some money and purchase them through eBay rather than B&H Photo (my usual vendor).