Canon EF 300 f4 or EF 100-400 f4-5.6
 

Im am buying a Canon EOS 40D and I want a walk around lens that I can use for wildlife photography especially birds. I am buying a 500 f4 for tripod work next year. I have read mixed reviews about the 100-400 but none about the 300. However, the extra 100mm reach of the 100-400 would be useful. Both are L lens and both have IS.

Any useful advise would be greatly appreciated
Michael

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.

If 300mm F/2.8 was an option, I would vote for it as it is one of the best lenses Canon ever made. Goes very well with TC 1.4x and also almost flawlessly with 2x to give you a 600mm at decent F5.6

With the two options you gave, 100-400mm is the winner without doubt. Problems or rather difficulties; as Christine said, It is an old lens and probably will be replace by Canon soon because of its blowing in and out while zooming and not being a whether sealed lens, it has the tendency to suck in dust. Not a good option especially with digital sensors, of course dust removal system of your 40D will take a good care of dust to great extend.

Going by your requirment..
..I would suggest 100-400. It's light to carry around. It's sharp and does justify it's cost.

and what aperture does the 100-400 mm have at 300 mm?

I have recently bought a 300m F4 and I am very happy.

Its an f 4-5.6 sadly, not straight f4.

If you're thinking about the 300mm as a 'walk around' have you thought about the 70-300IS? I own this and while not claiming it is up there with the 300, does make for a good tele/walk round, for the few times times I 'need' this FL.

The 100-400mm is f4.5-5.6 and at 300mm its f5.6 ive just checked.Great lens for walk about .

I think first you can review the original size photo by the 2 lens on 40D, then you will find the photo quality of the 2 lens.

40D + EF 300 f4
http://english.sellpower.net/modules..._mm=0&B1=Query

40D + EF 100-400 f4-5.6
http://english.sellpower.net/modules..._mm=0&B1=Query

I have found myself in exactly the same position this month. What I have done is to hire the 100 - 400 lens from lenses for hire to test first and at the end of the month will hire the other. That way you can get the feel for the lenses without spending huge amounts and then purchase the one you want from a more informed position. :)