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FlickrAnswer

This is in reply to this Flickr discussion:
http://www.flickr.com/groups/nikond90club/discuss/72157627237258748/
Since my reply is rejected by Flickr (who sees spam in it!), I put my reply here.
(Note that this wiki is password protected due to spammers. If you want to edit it, you have to ask me).
Short links to the present page: http://sn.im/tnn9d , http://bit.ly/rhYXiB

Hi. I was also interested in bird photography and I chose first a D90 and a Sigma 120-400. I got a lot of AF problems and all my pictures were out of focus. Certainly my 120-400 was not a good one, but even after a 6 weeks stay in Sigma repair service, I got it back with always the same problem. I bought a Nikon 300mm F/4 and a Kenko x1.4 converter. It was better, but still not perfect. Finally I bought a D7000 in order to be able to adjust the fine focus.

I made a lot of tests. If you can read French, you can have a look at http://bit.ly/grKsCl (there is a link at the top of the page for an automatic English translation).
There are also plenty of tests with plenty of images (but images are not apparent, there are under the links) on this page: http://sn.im/tests_photo_gt
One of my last comparative test is here: http://tinyurl.com/6b8xml6 , and among all the images, this one is of interest: with optimised fine focus on D7000, the same church tower, at 335m, with the Sigma 120-400, Nikon 300, Nikon 300 & 1.4TC : http://sn.im/27tbbp

You will find also plenty of examples on this gallery: http://sn.im/sflhd (looking at the focal you can guess with which lens I took the picture: 420mm means Nikon 300 & 1.4TC; every focal between 120 and 400 and different from 300 means Sigma 120-400, and 300mm could be Nikon 300 or Sigma 120-400...).

My advices:

Sigma 120-400 suffers from inconsistent quality. I bought mine in December 2009. Some people have got good lenses, and some other very bad ones. If you have a bad lens, you can limit the damage with a body which has fine focus (D7000, D300, D300S), but it is a lot of work, and a lot of disillusions also, since the fine tuning depends both on the focal and on the focus distance. Here is a link to the Excel sheet where I have reported these tests. If you look at the first table, you will notice that I need to use a -20 correction (it is the maximum) and with this correction, the focus is correct at long distances whatever the focal, it is not good at 400mm and short distances around 5m (typical situations for small birds), and if I use very close distances (1.5 to 2 meters), I imperatively need to use 250mm (Front focus for shorter focallengths and Backfocus for longer focallengths).
In any case, it is soft at 400mm and F/5.6. At the long focals, you will need to use F/8.
If you choose a Sigma 120-400, try to buy it somewhere where you can try it before.

Nikon 300mm and 1.4 converter: you loose the zoom, and considering the number of pictures that I take with the 120-400 and with focals shorter than 400, I need the zoom! But it depends on the photographs that you plan to shoot. You also loose stabilisation. Note that I also get some (less pronounced) AF problems with this lens, and they are different with and without the converter...

I still do not have THE good solution. But from my experience, my best satisfaction is the D7000, for the fine focus, and also for the improvement in sensitivity. Note that you easily gain 1 stop compared to the D90 (and the D90 seems to be better than the D300S from this point of view). It means that a F/5.6 lens on a D7000 will behave almost as a F/4 lens on a D90 (considering the bird photography, where you are always at the limit low aperture / high speed / high iso), and you get a little bit more depth of field.

Sorry for this pessimistic point of view, but I expected much better from this equipment!