Longer focal length will be better for birds as Joel pointed out, but will the 600mm give you the longest reach with acceptable IQ? I studied the chart when the lenses were released and what I saw was very interesting. The new 500mm II with the 2x extender is showing a better chart than the current 500mm naked:








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If the new 500mm performs as well as the charts indicate, you will be getting acceptable at 1000mm. The 600mm at 1200mm is not to shabby as well but would be getting closer to what someone might find acceptable. If that were the case that the 600mm was not acceptable at 1200 but the 500mm was acceptable to you at 1000mm then the 500 would be the better choice.


Actually a comment you made about the 300mm, you said you wouldn't upgrade because you see no room for improvement. Owning the 500mm I can tell you it is on par with the IQ you get out of your 300mm. You might think about getting the old version and saving the extra $3000 or so. I find the 500mm very acceptable with the 1.4x at 700mm but not acceptable with a 2x.





Good Luck