I have the Tamron 28-75. I'm starting to be a wee bit suspicious of it, I've gone back and pixel peeped my shots and it looks like it may not be quite as sharp as my Canon 70-200F4L (a very highly regarded lens).
However the equivalent Canon L lens for that range is nearly $1000, while the Tamron cost considerably less. And truth be told I use that range less, so perhaps it makes sense to pay less for a lens that covers it.
Still it's a good lens, no regrets.
I have done a bit of research into both of the lenses you're considering.. a lens in that range is probably going to be my next lens purchase. Although I'm considering a 400mm prime as well.
From my reading of the forums on dpreview.com I can say that the Bigma has a wide fan base and a reasonably good reputation, debated mostly only by Canon L lens loyalists. It's acknowledged that it's not sharp wide open, and at 500mm its widest still isn't that wide. So you need a lot of light, or a tripod or monopod, or excellent hand holding technique.
I still find it interesting that Sigma has an 80-400 with an image stabilizer on offer for the same price as their 50-500 which at least has the advantage of having their HSM autofocus mechanism.
For myself I want all the reach I can get, but boy.. an optical stabilizer would be hugely attractive, especially since so far I've been almost exclusively a hand holder.
I've seen comparisons of full crop (1:1, no resizing) shots taken with.. actually I'm not sure if it was the Bigma or the 80-400, but it was compared to Canon's 100-400 lens, an L lens with optical stabilization that costs $1600.. no wait, I just looked it up, it's down to $1400, or $1300 with their current rebate offer.
Anyway, I know you're a Nikon user but I'm not, I've been eying those lenses in comparison to what I have available as a Canon user. And whichever it was, the Bigma or the 80-400, compared quite favorably to the considerably more expensive (at the time) Canon 100-400. I'm sorry I can't be more specific but it was a while ago and I was watching both lenses.
I've seen people discussing the Tamron 200-500, but I don't think I encountered many if any owners. Some reviews suggest it's potentially a good lens, photozone.de certainly had positive things to say about it, but.. well let's just say that I'm glad to have found that the Canon 100-400 came down in price, that might swing my decision back to Canon L glass and save me from having to decide between the Tamron and the Sigma. Even if I lose 100mm in the process.
In any case I look forward to seeing what you pick and what kind of results you get. I hope this means you'll be bringing back some nice wildlife shots for the non Disney category.
