Harvey, Asher, Don,
The Canon 300 f/4 IS lens focuses pretty close on its own. When that is not enough, my first choice is usually to add the 1.4 teleconverter, so that I can keep the same distance, but get the subject larger in the frame. The extension tubes work, too, but they make you very limited when dealing with moving subjects going through difficult terrain (uneven ground, puddles, brambles, poison oak/ivy/sumac, stinging nettle, etc.), because they give you a very narrow range of distances from the subject in which you can achieve focus. This is just a rough estimate, but I'd say less than a foot of depth from nearest to farthest, all somewhere in the vicinity of four feet away from the subject. When necessary, the 300 works quite admirably when stacked with both the teleconverter and the extension tubes, provided you stack them in the proper order: the extension tubes next to the camera body, followed by the teleconverter, followed by the lens. (The autofocus does not seem to function as well, when the teleconverter is next to the camera body and the extension tubes are next to the lens... which is unforunate, since the extension tubes would have a more pronounced effect next to the lens.) The 300 stacked with extension tubes and teleconverter is the combination I used all day, today, on a butterfly photo excursion.
Mike