Hi, Having a little trouble with focusing at long distance with birds as the subject. I use a Canon 70D and a Canon 400mm F5.6 lens and wonder does the distance the subject is away from the lens cause focus issues? Close up birds within 15 to 100 feet shot hand held I get sharp images but when the bird/s are in the 3,4,500 feet or more away even when I use the centre focus option and rest the camera on the car side window with a bean bag seem to be OOF, not sharp around the eye area although focused on the head. Camera and lens have been calibrated with FOCAL. Any suggestions please? Thankyou, Russ.