It's not something new to the 40D. The fact is that if you use multipoint AF, the camera will simply need to guess as to what you want in focus. How can it possibly know where in the scene you want the focus to be placed? That's how it works on my 20D.
I've never understood:
A: How multipoint AF algorithms work - just pick the nearest item? Choose a distance that splits the difference between farthest and nearest? Choose a distance that has the most points giving the same reading? Geeze none of that should work very well
B: How anyone can imagine that such a system ever could work.
Unless the camera can read my mind and select the point that lies on the place in the scene where
I want to focus, I just don't get it at all. And that wouldn't be multi-point AF, it'd be mind-controlled single point AF.
Needless to say, I either focus manually or select the AF point by hand. Effectively, manual selection of the AF point IS mind-controlled single-point AF, I guess. And manual focus is even faster and better in many situations - it's direct mind-controlled focus with infinite "points".
Multi-point AI-Servo is an exception, I guess, because it's supposed to lock onto the center point first and then hand-off the focus to another point if the subject appears to move within the frame. Still pretty dodgy-sounding if you ask me, but some people say that it actually kind of works. With more points, and with color and contrast detection, the new Nikons may show us a way for AI-Servo to really work well with multiple points. But I really don't see how it can work well with so few points as we've got in these current XXD bodies.
Am I missing something, or is multi-point AF with so few points sort of - well - pointless?
Please note that I'm not knocking the 40D or any of these bodies. To me, they do exactly what I'd expect them to do. Really, it's quite amazing and I have NO complaints about my 40D's AF performance. But I simply do not expect magic or supernatural ability from the system. Thus, my (I think) more realistic expectations of the system lead me to be more satisfied with it.
In single point mode, my 40D focuses dead on within the limits of the actual area being "viewed" by the AF "points" and all of the other constraints of the system.
I'd like to read the review(s) that knock the 40D's multi-point AF performance because I'd be curious to know what it is that they expected the system to do. Maybe my expectations are unreasonably low.
The one thing I have read on here that I believe is that the 40D is faster. And that extra speed causes it to abandon focus lock and re-acquire again quickly if the subject is moved away from the focus point in the frame while you're in AI-Servo.
So it'd be nice to have an adjustment to provide a bit of a delay before the camera abandons focus lock and starts searching again. That would make life a lot easier for BIF and sports shooters. Maybe that's what you're seeing. And I agree that I can see how this could be a problem, but not really related to multi-point directly IMO. As I understand it, the 1-Series bodies do have an adjustment for this. I wish the 40D had it too.
I have not done a lot of BIF shooting so this extra speed has not been a problem for me yet with my 40D versus my 20D. And with all of my lenses, I have to say that my 40D focuses right on the money. And it's noticeably faster than the 20D was. So for me, the 40D has been a good improvement in AF performance over the 20D. But again, I have not done much BIF shooting, so that's one situation I cannot comment about from personal experience.
I think you'll probably be happy with your 40D and the new lens. But for BIF shooting, that might be where you'll find it to be "too fast". And it's always possible to get a lens or body that is not calibrated properly. But I think it's more rare than one might imagine based on reading these forums.
Good luck with the new lens and the camera. I think you'll most likely be very happy.
--
Jim H.
