I currently own a 5D mkI (12.8 MP) and a 70-200/4IS. I don't shoot wildlife a lot, but when I do, it's while travelling the world and 200mm on FF is not enough. I think that for my shooting 400mm is OK.
While travelling: this is in my bag:
One or two FF bodies, 16-35II, 24-105, 70-200, 580EXII.
I've been thinking about various options:
- 400/5.6: yes very impressive IQ, but too bulky for travel, no IS and no zoom
- 100-400: so so IQ, not fast, won't give up my 70-200 for it.
- 70-200/2.8mkII w/ TC's: I would trade my 70-200/4IS for this lens and I can give it a lot of good use it at home (weddings, family portraits, events, stage, etc.)
I rented the 70-200/2.8ISmkII recently to shoot a wedding and I tested a Canon 2xExtenderIII (the new one) on it. I was a bit disappointed. Yes it is quite sharp, but there was not much contrast left and weird OOF areas in the immediate background.
So I was thinking, hopefully you can follow and comment:
1. For wildlife images, 10-12 MP is enough for me
2. I will get a 70-200/2.8ISmkII probably anyway and sell my 70-200/4IS
3. I will most likely get a 5DmkII anyway (for other reasons) to become my main body and keep the 5D as 2nd camera for weddings and stuff
4. A 1.4xTC makes IQ suffer much less than a 2xTC
So: why don't I get the 5dmkII and the 70-200/2.8mkII and get a very good 1.4xTC to get to 280mm. Then, in PP, I crop the 21MP image to 10-12 MP, allowing for another 1.4x "magnification". No interpolation involved, just quality optics and pixels.
From reasoning, I expect the final IQ to be better than that from a 2xTC (even a new III version) on my 5DmkI, using all the pixels. Though I know theory and practise can be miles apart.
Edit1: after rereading, just to make everything perfectly clear, here is what I want to compare:
5D (mkI) + 70-200/2.8ISmkII + 2xExtenderIII (12.8 MP)
5DmkII + 70-200/2.8ISmkII + 1.4xExtender + crop to 10-12 MP
Edit2: I would like to add that I am determined to stay on FF. I see a lot of folks on this forum shooting APS-C and that makes a lot of sense if birding is your main thing. For me, it is more general photography and I found that FF bodies fit my needs best. I probably wouldn't buy a APS-C just for shooting birds. That said, if anyone thinks a 40D+1.4x+70-200/2.8ISmkII beats a cropped 5DmkII, I would really like to know about it. Used 40D's don't cost that much nowadays...
Hope you will comment.