Looking to go longer than 200mm for under $1000

I'll add my $0.02.

The Sigma 150-500 is nice and versatile. It's supposed to be pretty good at 400 mm, even wide open. Not sure how it does at 500mm wide open. Bad news is that wide open is f/6.3, so you kill a lot of your low(er) light situations, especially if you are stopping it down.

The cheapest (and in my opinion, best) route is to get the Nikon 2x III. I have the 70-200 and 2x III and get very nice results. You end up with a nice zoom lens with max aperture of f/5.6. Not that you can compare it to the Nikon 200-400 f/4, but it's the same range and only one stop of light worse ... but considerably cheaper since you already own the 70-200.

I wouldn't get the 80-400 ... the 70-200 + 2x is faster autofocus and probably better IQ. Can't speak from experience, but that is what I read.

Take a look at the Nikon 300 AF-S and non AF-S versions. You can pick up the latter for under $500, pair it with a Kenko 1.4x, and be good to go! Below are some images that I have taken with that set-up: http://www.flickr.com/photos/mpalaviccini/sets/72157631550833924/

If I were you, I would opt for the 1.4 and 2x teles and work with those. That way you will have a 280 f/4 and a 400 f/5.6 (although not primes, they will produce stellar results). If you ever upgrade to a prime lens down the line, you will still have the teleconverters.

Hope that helps!

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Miguel http://i91.photobucket.com/albums/k307/Miguel_Palaviccini/gatorengineering.jpg