Michael if you pop across to BF(Birdforum.net )there are lots of threads re the various lenses.I have used both of these lenses.I still have the 100-400,the first lens I purchased around 4 yrs ago.You mention a "walkabout lens".You have the option of altering your focal length with the 100-400.Also it slots down to fit into a smaller bag,as opposed to the 300 fixed focal.Both lenses can be used with an extender(canon 1x4,will autofocus with the pins taped),so you can easily be on the move with equipment which will focus up to 650mm.More, as you are using the 40D,which is not full frame.The 100-400 is very versatile.You can use for landscape type shots ,anything which which needs closer focus than the 300mm.So when you purchase your 500mm,you will still have a lens which will focus closer if needed.I went to Martin Mere last week to meet up with some BF members,and some of the birds we saw from the hide,were so close,a fixed focal lens,would have been of no use whatsoever.
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