I consider extension tubes essential gear. The hollow tubes fit between the lens and the body, and they allow you to focus very closely -- closer than the lens is capable of without the tubes. For example, this vervet monkey from Kenya was sitting just eight feet away from the land rover. I wanted to fill the frame, but my 500mm lens wouldn't focus this closely. The minimum focusing distance is about 15 feet. Therefore, I placed an extension tube between the lens and the camera body, and now I could focus on the eyes of the monkey. Extension tubes won't let you focus to infinity, but in this case that didn't matter.