I tried a 36 mm Kenko extension tube on my Nikon 70 - 200 mm VR lens to get a feel for how extension tubes work and what they are good for.
What I learned is that the extension tube reduces the minimum focus distance of the lens from about 6 feet to about 1 foot.
But in so doing the useable distance from the subject is limited to a range of about 1 to 2 feet.
Furthermore, with the 36 mm extension tube, the lens can only be focused at or near the 200 mm setting.
That said, the results are interesting in that I was able to shoot full frame at a distance of a foot from the subject and get a decent depth of field at F 22 as seen here and in the next slide.