Sigma OS 50-500 vs 150-500 vs 80-400 vs 120-400

You are being confused as you are looking at the discontinued lenses like 80-400 or 170-500 too.

However current lenses too are many, 120-400, 150-500 50-500, all stabilized i.e. OS. There's a 120-300 2.8 OS too but that's costlier and heavier.

I have had the 50-500 OS for some time now, and I think like all such lenses, it needs a bit coaxing to perform at it's best... trying to get to know this big guy now

--
http://www.flickr.com/photos/subhrashis