Extension tubes are basically spacers. They come in a set of three, such as the Kenko tubes pictured below, and they fit between the lens and the camera body. They don't have any glass, so the quality of your prime lens isn't affected in any way. They can be used with a macro lens, such as a 50mm or 100mm macro, or they can be used with a telephoto lens such as a 70-200mm zoom or even a 500mm super telephoto. Their primary function is to enable you to focus closer, to fill the frame with very small subjects.
When you place one or more tubes between the body and a telephoto lens, the minimum focusing distance is reduced. An example is the photo of a red-bellied woodpecker.
This bird visited the feeder outside my office window, and it was only 10 feet from my desk. I wanted to use a 500mm f/4 to fill as much of the frame as possible with the bird (this photo has not been cropped), but the minimum focusing distance of this lens is about 15 feet. By placing an extension tube between the lens and the body, I was able to focus to 10 feet. This picture was taken right through the glass window.
Many photographers who enjoy doing macro photography prefer a telephoto macro, such as a 180mm macro, as opposed to a 50mm macro lens. Telephoto macros allow you to focus on small subjects from several feet away, instead of several inches, and the background goes out of focus more readily. Completely undefined backgrounds are usually more desirable in closeup work because that forces all of the attention on the subject as you can see in the picture of the caterpillar.
If you use extension tubes on a medium telephoto, you instantly have a telephoto macro. That's why I often use an extension tube(s) in combination with my 70-200mm f/2.8 lens. This gives me a telephoto zoom macro!
Notes from the Editor
Article by Jim Zuckerman. To learn more about photography, explore the many online photography and Photoshop classes offered here at BetterPhoto.com.