Hi All. I have a Canon650D and have been using a EF 70-300 1:4 - 5:6 IS USM lens for distance shots = particularly wildlife. I would like a longer lens, e.g. up to 500 or 600 mm, to capture wildlife that is further away. The canon lenses seem very expensive for this range (for me at the moment) - can anyone recommend a cheaper one, e.g. Sigma ???