[RikWriter]

Okay, here's the situation.
Most of the photography for which I need a long lens is wildlife shooting, particularly birding but also deer and elk and such when I am on vacation out west.
I have been shooting with a Canon 10D, but I just upgraded to a 20D.
I HAD been using a Sigma 50-500, and it was working out all right for birding...but then I took it with me on vacation. I am not a small man, and I don't have a problem carrying a bit of weight, but carrying that Bigma in a backpack on several hikes of more than 3 miles convinced me I need something lighter. What also helped in that decision was two different experiences shooting elk. The first, I was close to my car and able to use a tripod and the pics turned out awesome. The second, I was far away from the car and only had a monopod. Those pictures were unusable.
So, I need something lighter that will still get me some serious reach.
I've narrowed the contenders down to these three:
1)Canon 400mm f5.6L USM
2)Canon 100-400mm f4-5.6L IS USM
and
3)Canon 70-200 f2.8L IS USM with a 2X extender (I already have, and love, a 70-200 f4L, but I wouldn't care to halve that f4 by using a 2x extender on it.)

So, which of these meets my requirements the best in your opinion? Which is a better buy for the money? Could I get away with getting the non-IS 70-200 f2.8 and using the extender, or would I be better off with the IS for monopod and hand-held shots?
Also, which of these would be lighter? And how much longer is the 400mm prime than the collapsed zooms?
Finally, are there any possibilities I am overlooking?
Any help would be appreciated.
TIA

[EnglishBob]

Firstly forget the extender and the 200, very soft, and you loose too much light to be useful, I've tried it.

For wildlife you would be better off with the 100-400 in my opinion, the IS will be the only way a 400 is usable handheld in most wildlife siuations. I've used a friends 100-400 on a couple of trips and it is impressive.

That said, it is only around 1lb lighter than the Sigma you have been using.

The Canon 400 f/5.6 is the lightest option at around 1.5 lbs lighter than your Bigma. But finding youeself locked at 400mm can proove a problem shooting wildlife where moving isn't always an option. The Prime is 2.7" longer than the collapsed zoom. 10.1".

I've still choosing option on which to purchase myself, the Bigma or the 100-500, need to get my hands on a bigma for some test shots.

Also moved your post to the correct forum

[RikWriter]

Firstly forget the extender and the 200, very soft, and you loose too much light to be useful, I've tried it.

For wildlife you would be better off with the 100-400 in my opinion, the IS will be the only way a 400 is usable handheld in most wildlife siuations. I've used a friends 100-400 on a couple of trips and it is impressive.

That said, it is only around 1lb lighter than the Sigma you have been using.

The Canon 400 f/5.6 is the lightest option at around 1.5 lbs lighter than your Bigma. But finding youeself locked at 400mm can proove a problem shooting wildlife where moving isn't always an option. The Prime is 2.7" longer than the collapsed zoom. 10.1".

I've still choosing option on which to purchase myself, the Bigma or the 100-500, need to get my hands on a bigma for some test shots.

Also moved your post to the correct forum

I've held the 100-400 at the same time as my Bigma and it sure felt like more than a pound lighter. I dunno, maybe the tripod mount collar wasn't on it at the time, I can't recall correclty right now.

[EnglishBob]

1850g for the Bigma, 1300g for the 100-400.

[RikWriter]

BTW, I went with the 400mm f5.6L. Found one in mint condition for $850 and couldn't turn it down.

[EnglishBob]

Great choice... I can't decide between the canon and saving a few hundred and getting the Sigma 50-500.....

[RikWriter]

Great choice... I can't decide between the canon and saving a few hundred and getting the Sigma 50-500.....

Well, the 50-500 is a great lens---IF you're comitted to bringing a tripod along whenever you use it.