Här är ett citat från intervju som dpreview.com gjorde med Canon under Photokina:
Q: One of the most common complaints we've seen about the 5D Mark II is that it still has the same AF system as the original 5D. Why is this?
A: "Firstly the market's evaluation of the 5D's AF system has been very positive; there have been no complaints from users, with everyone saying it's very good. Given that, to a certain extent, we think we shouldn't change it. And also there's some limitation with size; the AF sensor in the 50D is very big; the one in the 5D is much smaller. If we wanted to have all cross-sensors in the 5D Mark II, it would mean we might have to sacrifice the compactness of the body. It's all a question of balance of features and benefits."