How do multi-point AF really work?

Cuong

Nowaday all DSLRs come with multiple autofocus sensors (9-51). Proponents of multi-point AF say it helps to track moving subjects better. I read the newer Nikon bodies even have 3D focus tracking. The specs and the marketing features all sound wonderful but how does it really work? I have a Canon 40D with a 9-point AF. Let's say I want to focus on a track athlete getting ready for her high jump and continuously track the athlete as she runs, takes off for the jump and lands. Should I have all the focus points or just one focus point enabled? With all focus points enabled, how does the camera know which point to focus with a subject that changes from static to dynamic? When a single focus point is used with servo mode, I would lose focus on the athlete as she moves around and the focus point falls on the background.

Cuong

ziggy53

Briefly, AI-Servo focus mode takes focus samples of the subject and then tries to predict rate of change according to the samples. If it correctly predicts the rate of change then it can position focus correctly at where the subject will be at the time of shutter release.

Obviously, accuracy depends on the variability of the subject and how the camera is oriented during subject motion.

Remember too that with AI-Servo you need to activate the process before the camera will start sampling. This is usually done with a half-depress of the shutter button. Treating the AI-Servo like Single-Shot and just slamming on the shutter button doesn't usually produce desirable results.

If you select a single focus point and maintain the subject under that point, the camera will generally have better samples than multi-point focus selection and letting the subject move through the field of view (IMO). If you use multi-point focus selection the processor dedicated to the AI prediction would seem to have more processing to endure than a single-point selection and accurate tracking by the photographer.

Plus the center focus point, especially if it is a cross-type, also tends to be the most accurate of all the points. This is because of the construction of the electronic rangefinder and its alignment with the axis of the lens. Longer telephoto lenses tend to reduce the problem because they usually have a longer convergence angle from the rear element.

ziggy53

BTW, here is the definitive document about Canon's AI-Servo autofocus:

http://photonotes.org/other/ai-servo.html

Cuong

Ziggy, thank you for the explanation.

I went back and read the instruction manual for the 40D and found the answer on page 77, which states: "When the AF point selection is automatic, the camera first uses the center AF point to focus. During autofocusing, if the subject moves away from the center AF point, focus tracking continues as long as the subject is covered by another AF point."

Cuong