If you are into macro photography, which normally means having close to 1:1 magnification, sometimes it can be prohibitive as macro lenses are very expensive, costing thousands of dollars. However, with a cheap manual extension tube, a good prime with focal length of 50mm (or less) can be very effective and with excellent results.
This video show how to convert a standard 50mm lens into a macro lens using manual extension tube. It is better to use a prime (or zoom) lens with focal length between 28mm to 50mm. Shorter focal length lenses normally have short working distance, ie, distance from lens to subject, as you increase the length of extension tube. Longer focal length lenses can be used, too, but they require longer extension tubes to achieve similar magnification.
One aspect of the video is to deal with the fact that Canon auto lenses keep aperture wide open when it is dismounted normally. The trick to set the right aperture is to press and hold the DOF preview button and then dismount the lens. Note this trick COULD be RISKY so please be cautioned, though so far, it has been done many times with many lenses without harm to camera nor lenses.