Thank you for all your views on my question which long lens with Canon 5D. I have decided to sell my Canon 70-200 f2.8 Lens and Canon 2x converter and buy either a Canon 100-400 IS or Sigma 50-500 Lens with a Canon 1.4x or Tamron 1.4x converter. I am swaying more towards the 100-400 because it is lighter and has IS but the sigma would give me the extra 100mm length which one should I go for to get that buzzard hovering in the sky?? Also, does anyone know whether either of the teleconverters would work on auto focus with either of these lenses please?Look forward to hearing from you :h?: