My Review of the AWESOME Sigma 120-300mm f/2.8 "S" Lens
If you can afford this kind of telephoto lens and you're in the market for something in this price range that covers these focal lengths, then you have to consider the new Sigma 120-300 f/2.8 "S" as a viable alternative. If you can live with the 7.5 lb weight and the 105mm front filter thread then the Sigma is about 97% of the first party offerings from Canon and Nikon for about 60% of the price, oh, and it's more versatile too.
Great Price on the Sigma 120-300mm f/2.8 "S" from B&H
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