That lens (I have one) does excellent with either a 1.4 or 2X converter. Don't scrimp. Buy the Canon converter (extender). I got a 2X used, I think I paid about 150 for it. New ones now are 450. That combo is nearly as good, is some cases "as good" as t he 10-400L lens which I also own. You should seriously consider finding yourself a used 40D, at least. They have magnified "live view" which makes this kind of focusing much much easier. I was astounded at how cheap used 40Ds have become. (I had two) I sold one to a friend of mine, and soon learned you can find them for as little as 300 bucks
