I've owned one of these lenses since I received my first DSLR, a 50D, in December 2009. Mostly, I've used it for Imperial Hill shots at LAX though I've taken it to other airports as well. It's done a fairly good job but as the summer wears on I've noticed it's gotten harder to get crisp shots with it, especially for small aircraft (or most recently I was attempting to photograph a loon on a lake with it.) There's a lot of possible reasons, and I'm very much an amateur, so I'm hoping some more informed people might be able to clarify a bit as to what's frustrating me here. Here are the potential causes I can think of so far:
-I've been using the wrong aperture settings and not realized it. During the aforementioned loon-on-lake effort, I noticed that moving up from f6 or 7 to f10 or higher made things a bit crisper, though not completely so.
-Summer air in Los Angeles is just difficult. There is more smog/haze/etc. That might be making focusing harder I suppose.
-I'm using the wrong autofocus setting. Typically I use "one shot" mode with only the very center autofocus point in use. I know the 50D has an option to autofocus a moving target but I haven't done much experimentation with it.
Anyway, thanks very much for any help anybody can provide.