Autofocus technique question

<p>I take lots of photos of cross country races, using my trusty Canon 20D. At the finish, it often comes down to two runners racing side by side, with me trying to shoot them straight on. If I make a balanced composition with one runner to the left of center and the other to the right of center, the camera tends to focus on whatever is between the runners, which is generally way behind them. This is true even though I have all the focusing spots activated; the focusing system apparently is biased toward the center spot. (I am, of course, using AI Servo mode, so it's not a matter of the runners running past the point of focus.) Is there some technique to work around this?</p>

<p>Set your focus point to be only left (or right), assuming they will be roughly parallel?</p>

<p>Have you tried putting the center focus point on one of the runners and starting the AF system (half-press shutter or "*" button) well before you want to take the first shot? The servo AF algorithm is suppose to try to track focus on the subject it first saw in the center point even if the center point isn't on the subject anymore.</p>

<p>I have every good luck with this technique shooting well-separated subjects (ex. fast-moving planes) even if I don't keep the center focus point on the subject all the time. It may fail if the center point is off the subject for an extended period of time.</p>

<p>Mendel: That would be ok if I had a bit of time to prepare, but I'm just faced with a constant stream of runners, many, if not most, of whom I want to center in the photograph.<br>

Geoff: I'll give that a try. It would make sense to design the algorithm that way; this must come up all the time in action photography. Maybe I'll get my son to run toward me for a few practice times to see if I can get the hang of the technique.</p>

<p>I think Geoff's suggestions are very good. I really use servo infrequently, mainly for mobile grandkids, but this thread motivated me to read up in my manual (the 5D body but similar I think) some: servo will hand-off the focus from one focus point to another as the subject shifts in the frame. Interesting!</p>

<p>Maybe you could get some volunteer runners (family, friends) to test servo focus technique?</p>

<p>You could prefocus manually at a point they will pass, I do this often with bikeraces. This is great for composing the background too.</p>