Sat 30-Mar-13 12:50 PM | edited Sat 30-Mar-13 01:20 PM by KnightPhoto
I'll start by saying that for non-flying birds in my experience, fieldcraft to get yourself closer to the subject is going to have a bigger impact than glomming on ever longer TCs onto ever longer lenses. They get difficult to handle in terms of narrower filed of view, vibration management, shutter speed management, narrow DOF and getting the primary AF sharpness EXACTLY where you need it (via AF fine-tune), higher ISO management, and rather large effect on AF speed (which even for non-flying birds can be a factor).
I sold my TC20Eii a long time ago as I no longer needed it. Mostly it was my stretch tool when I first started with my 70-200 VR1. But I did try it on a few occasions when I later got my 500VR and due to the above factors didn't find it lead to better real-world results.
Since then I've stuck to TC14 and TC17 and tried to improve my fieldcraft. Getting closer also eliminates problems with atmospheric conditions that cause softness. With super telephotos this is a very real factor.
What kinds of birds? There is different fieldcraft depending on subject type. E.g. Shorebirds, Songbirds, Feederbirds, Waterfowl, Owls, Raptors, etc. And a lot of knowledge goes into knowing where and when to go to find certain targets and within those locations where the favourable photo ops are possible.
A lot of the best bird photography involves setup and fieldcraft.
As far as 500 vs. 600mm I have some thoughts (from a 500VR shooters perspective) in my blog below. The only caution is that was written from a DX (D300) shooters perspective. Nowadays with FX, it is tougher to get that last bit of closeness with the 500 and 600 would be better. Only problem is a 600 does NOT handle like a 500. Pretty much night and day difference in ease of handling a 500 vs. a 600. On the Canon side they have a new 600 that is as light as their old 500. I would love to see that option in the Nikon side. The new Nikon 800mm uses fluorite and that appears to be a key part of the puzzle in getting the weight down. So if Nikon were to apply that technology to a new series of 600/500/400, we would all be better off.
Also, because of these difficulties, a D400 still makes some sense for those occasions when light is plentiful. I still intend to get one.