I kept feeling like my 24-70mm f/2.8L was just a little off, but it wasn't enough to really cause me concern. Some images were spot on focus wise and others seemed ever so very slightly out of focus right where I wanted the focus to be (mostly noticeable at f/2.8 ). So I downloaded a focus test, as a sanity check. Sure enough, the lens was ever so slightly (one micro-adjustment) front focusing. So I went into my 50D's menu system and brought up the micro-adjustment for the 24-70. I gave it one click of "back" adjustment and took another test shot. Bingo. I had the same feeling about my 40D but I never tested it. It too was ever so slight, but I really didn't want to send it into Canon just to be sure. It really wasn't enough to be concerned about. I would say the micro-adjustment of the 50D is well worth it. Just one more reason I'm happy as a lark I upgraded.