Paul Jolly Photography Chapter 15

Extension tubes

A lens normally focuses from infinity to a close point (this is different for different lenses).
Adding an extension tube to the back of the lens changes the focus closer to the camera, bringing back the infinity focus and more importantly bring the close focus much closer.
More extension brings both focus points even closer, this means that the object being photographed covers more of the cameras sensor at the cost of much less focal range.
Extension tubes are not for hand-held use unless you have very very steady hands, it is better to use a micro-adjustment positioner on the camera if you wish to take this kind of shot. For more information on how this works see the chapter on back focus distance.
CH15 extension tubes seperate