Hi All I own a Sony Alpha A100 and have the 70 - 300mm zoom lens. However, as I am intersted in bird watching, I wish to get a larger zoom lens. My local shop (Jessops) has recommended a Sigma 170 - 500mm f5-6.3 (cost £549). Budget is limited (hence the Sigma), but if someone recommended something better, then I'd have a serious re-think. Any suggestions?