hello everyone, I'm completely new to the world of photography and to this website. I purchased my first camera (Rebel XSI) in October of 2009 and just recently purchased a Canon 7D which I now exclusively use. I have noticed a bit of improvement with the pictures but I seem to have a hard time focusing in on my subjects. I don't know if its just the lens I use (Sigma 170-500mm which I borrowed from a friend) and a kit lens which came with the 7D (28-135mm USM w/ IS). I tend to take the majority of my pictures with the Sigma because most of the subjects are and I can't get all that close. I have decent success with the kit lens for flowers and insects. I want to take pictures of the birds in flight but i just can't get it to focus or be very sharp. Are my problems due to the lenses, my focusing or my camera settings? Any help that you may be able to offer will be greatly appreciated. (Below are some examples; all pics were taken on a sunny day with no clouds).. 1. 2. 3. 4.