If you want a lens primarily to photograph birds in flight, the 400mm f/5.6 is better by far than either the 300mm IS alone or the 300 IS with a 1.4X tele-converter.

Several factors make the EF 400 far superior to the EF 300mm IS for flight shooting.

1. The 300 IS has such a small minimum focusing distance that, even with the distance range limit switch set to the "far" setting (3m to infinity), initial focus acquisition takes a bit longer than with the 400 lens (even with the image stabilization feature turned off).

2. With the IS feature turned on, initial focus acquisition is a bit slower still (but still adequate). 3. With a 1.4X tele-converter in place, initial focus acquisition will be slower than with the prime lens alone for all autofocus lenses, and the 300 f/4 IS is no exception. Note: Pre-focusing manually with the 300 IS lens significantly reduces the time needed for initial focus acquisition.

If you want an everyday bird photography lens, the EF 400mm f/5.6L lens is still the better choice (unless you absolutely refuse to use a tripod). Why? For bird photography, the general rule is to choose a longer, slower lens over a faster, shorter one.

But (and this is a very big "but"), if you want a highly versatile intermediate telephoto lens that can be handheld at relatively slow shutter speeds, can be used from a boat without a tripod, is superb for sports photography, makes (with the addition of an extension tube or two) a superb macro lens that offers lots of working distance, is fabulous for shooting tame birds and other wildlife, is a great safari lens, and, is a good lens for photographing birds in flight and in action, then the Canon 300 f/4.0L IS lens might well be perfect for you.

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