I am looking for experienced advice on a 400mm focal length purchase for bird photography. I am looking for the best IQ 400mm under $2000 (used). I have read good things about many lenses but not too much info available on lenses with 1.4x or 2x converters. I think this is what it comes down to (let me know if there are other contenders): 300/4 with 1.4x, or 70-200/2.8 with 2x, or 100-400 IS, or Sigma 100-300/4 with 1.4x. Image quality is first priority and focus speed second. Much thanks for your help.