Canon 100-400 is lens

At what aperature (shooting at 400mm) is this lens the sharpest? I am using it on a 20d and would like to use it for birds mostly.

I find one to two stops down works well. Even at 5.6 it's not bad. I happen to have a well calibrated copy. I sent it to Canon for cleaning and calibrating and it, to me, is great. I love the versitility. It is often used as my work horse lens an often used with extension tubes to shorten the closest focusing distance for semi macro of larger subjects like Dragonfiles.

Thanks for the feedback Ed.

That was funny. I read is as a is and not IS.

If you really want this lens to be its sharpest, back off slightly from 400mm. I owned one for many years and found it much sharper at about 380mm.

On my 5D, I find that f8-f13 is the sharpest.

My copy is sharpest at f7.1 to f8 and about 350mm. I most often shoot it on the 20D.