Ef 300 is ism
I have owned the 300mm f4l is for a few years. Initially I used it on an XSI and noticed a tendency of the lens to slightly front focus. The lens is very sharp, but does have that tendency. Shots look sharp, but when I carefully look at the images on screen I can see where the branch just in front of the bird, or the foreground immediately in front of the subject is a bit sharper. There is no micro focus adjustment in the XSI and since images with my other lenses are sharp, I figured it to be a minor calibration issue with the lens. When I bought a T3i, my hunch was confirmed since the lens behaved exactly the same on that body. I just purchased a 5d Mark III. My first full frame. I just got through the manual and one of these days when I have a little time, I will enjoy making a slight in camera micro focus adjustment and see how this lens works. My guess is it will be bang on with a little bit of adjustment away from the camera. With micro focus available in your body, you should be able to make this type of adjustment. When you are dealing with lenses of this much reach, the depth of field can be extremely narrow, so it's likely not a major fault with the lense, and something that can be compensated for with mfa. Good luck!