I know this is an old thread but as I recently got the 300 f4 IS and alread own the 400 f5.6 I thought I'd dig it up (just in case anyone is still pondering). I've had the 400 f5.6 for just over a year and love it, the focusing is very fast and the image quality is excellent (alomst on a par with a 500 f4). People often quote the minimum focusing distance as a negative, but when shooting birds I have yet to find this to be an issue, even at MFD a bird like a robin will be almost frame filling. However for butterflies/dragonflies the MFD can be an issue, which is why I've picked up a 300 f4 (which is also handy at a feeding station). I used to use the 100-400 IS and although it's a good lens for me it would come in third after the other two options. The 300 f4 is definitely more versatile than the 400 f5.6, but if I could have only one it would be the 400... I reckon it's the best walkabout lens for birding available. Here's the reason I love the 400 f5.6...
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