When it comes to close-up photography you have several options; this article discusses extension tubes.  For close-up lenses please see here. As I have previously mentioned it is hard to beat a dedicated macro lens such as the Canon 100mm f/2.8 Macro for close-up photography.  However the expense and weight of an extra lens can make it too much of a luxury for many photographers; extension tubes offer a much more attractive alternative. Kenko produce a set of three tubes in 12mm, 20mm and 36mm sizes for both Canon and Nikon mounts.  These can be stacked in any combination to give the desired magnification.  They do cause some light loss, but as they contain no glass they have little effect on image quality.

The tubes are placed between your camera body and lens to increase the distance between the lens and film plane.  This allows the lens to focus much closer than normal.  The greater the length of the extension tube, the closer the lens can focus and therefore the greater the magnification.  The simplest analogy is to consider a projector and a screen, the projector being the lens and the screen the CCD/film.  Moving the projector further back results in a larger image on the wall.  Since the screen is the same size it only shows the centre of the image, which is magnified and darker since the image is spread over a larger area.

Kenko's tubes have the advantage of maintaining electrical contact between the lens and body, ensuring auto focus and TTL auto exposure remain functional.

It is worth noting that extension tubes prevent the lens from focusing to infinity. The focus range is greatly limited to a very close focusing distance.  Manual focus is also recommended since auto focus is often affected by the reduction in light reaching the AF system.

The shot below was taken with the Canon EF 50mm f1.4 and Kenko 36mm extension tube.  Depth of feild is very limited with extension tubes so it is usual to use a very small aperture such as f11.  However in the photo below I chose to use a wide aperture of f2.8 to give a dreamy soft focus feel.