Well, there are a couple of reasons. With the 40D the viewfinder is marginal for manual focusing. At f/8 or f/11 the viewfinder is even darker and it's hard to tell proper focus. Also with live view set at 10X I found that by focusing on the is the shoulder of the bird the feather detail help me get focus right. But at F/5.6 the head and eye would still be soft when the head was lowered at all. So the added DOF gives me sharp focus in the majority of the birds body. When possible I would focus wide open on the eye but still step down to get more DOF. Hopefully I'm making sense right now, the backs acting up and I had to hit the pain killers and muscle relaxents which leaves me a bit befuddled.