ok, I have the 400mm f2.8 IS and is a great lens but I would give it up in a second for one 600mm IS lens,the 400 mm is not enough when you talk about birds,and you cant believe how heavy this thing is,I can only handhold it to my eye for 30 seconds at max.so my advice is to go long and buy the longest one you can afford if you are talking only about birds.if you want to use it for sports ,the 400 is the better option here.


or buy a used 600mm IS and let