With new tele lenses with internal focusing (IF) we have other tricks to increase magnification.

IF lenses do not extend physically when focusing closer than infinity, rather, lens group(s) are moved back and forth within the lens barrel. This renders the tele more balanced and comfortable to use, and also allows to reach a shorter  minimum focusing distance (thus a greater magnification), than old tele lenses.

However, the actual focal length of IF tele lenses decreases markedly at short distances, and the difference between the nominal and the actual focal lengths can be rather great at the minimum focusing distance..

For instance, the recent Nikon and Canon 300 f4 AFS/USM (as well as the older 300 f4 Sigma Apo Macro) lenses can be focused as close as about 1,5m (the Sigma goes even closer), but at that distance their actual focal length is a mere 220-230mm. So, with an IF tele lens  you gain some magnification because you can go closer to the subject, but in turn you lose some magnification due to the shortening of the focal length

 HERE COMES THE EXTENSION TUBE: With an extension tube, at medium short distances  you can gain more magnification even without moving closer to the subject!

Why? Simple, because by adding an extension tube, even if you do not change the distance between film/sensor and subject, you must put the focusing ring to a greater distance to have the subject in focus in order to compensate the extension.  As a consequence, by focusing to a greater distance the decrease of the ACTUAL focal length will be less pronounced, so that the magnification will be GREATER than without the tube. So, by adding  an extension tube to your IF tele lens you can gain magnification even without moving closer than the minimum focusing distance.  Shots taken with a Nikon D200

I have taken some shots with my Nikon 300 f4AFS with aan without the Nikon PN 11 (a 5,2cm extension tube).