Originally posted by Lukinosity I have never understood how multiple point AF works. Do people just trust the camera to pick what they would like their focus to be? I like to tell the camera where I would like the focal plane. I always use single point.
I would think one would really need to understand how the AF system uses information form the various AF points for the camera to "decide" where to put the focus. Wil it pick the closest object? The closest object near the center of the frame? Or the focal plane that gets the most AF point hits? If one knows how the system works, THEN maybe more points is helpful. Otherwise, you may as well roll the dice.
What you're describing is what in Pentax world is called AF-A. As you said for the most part you dont really know what the camera will choose in this mode, however that isn't the only use of a camera with many AF points.
My most recent pentax was K-x which has pretty bad / simplistic AF-C, so I can only chime in on how I use predictive AF with 39 points on D600 / D7k (AF-C-3D in Nikon)
I usually always start with center point, I put my subject in center, and hold down AF. As my subject moves across the frame and closer / farther from me, the AF point moves and it's smart enough to know what was previously in the center (usually in my case a person's face) and keeps that object in focus as it changes distance and location in the frame. I dont have to recompose to keep the subject under only that one point.
I know there are other uses for it, but this is the way I use it. On my stress test I can track my son running towards me and get 85% keeper rate of his face in focus (yes most of it is crap, but one or two of them usually have a nice expression I wouldn't get otherwise)
So no, you're not necessarily playing dice, you're still in control, only your tool is a bit smarter now.