One of the reasons I picked up the 300 f/4 IS was because of its minimum focusing distance of less than 5 feet. When you add an extender (teleconverter) the minimum focusing distance doesn't change. So, with the lens and extender, I had a 420mm focal length lens which focused at less than 5 feet, resulting in a maximum magnification of .33x. This is more magnification than any Canon combination other than a macro lens and plenty close for most uses. The lens I replaced, the EF 400 f/5.6, has a minimum focusing distance of 11.5 feet and with the 1.4x only produces .18x magnification, without IS or autofocusing (due to the small aperture).
As I wasn't intending to shoot closeups, I didn't have a tripod and even with the monopod and image stabilization I needed to keep the shutter speed up to prevent blurring. So, even at ISO 400, I couldn't go with an aperture smaller than f/8. The picture below of hair-capped moss shows how narrow - or shallow - the depth of field is at that magnification.
|Canon EOS 5D, EF 300 f/4L IS, 1.4Xii Extender, ISO 400, f/8, 1/125 sec.|