Autofocus for action

Left Image: Minolta Maxxum 9, 200mm f-2.8 APO lens, Fujichrome Provia 100, f-2.8, shutter speed not recorded, continuous autofocus mode.

I see autofocus as an extra feature rather than an essential one. You can certainly take good photographs by focusing manually. People have been doing this for decades and continue doing it today. All my 35mm cameras are autofocus, but seventy percent of my photography is done in manual focus. I prefer to select my own focus point and check depth-of-field for optimum sharpness. I see no reason to use autofocus for landscape or architectural photography for example, when you have time to focus on your subject. For action photography, I prefer autofocus with  tracking. I'll explain how this works and how you should use autofocus for best results.
 

All currently made autofocus cameras have predictive autofocus. Camera can calculate subject's speed and direction to adjust focus for the exact time the shutter opens to record exposure. This ensures high percentage of sharply focused images. Older autofocus cameras did not have predictive autofocus. If you are switching from an old model to one of the newer cameras, you have to learn how to use  autofocus all over again, especially for fast moving subjects. I'll give you an example with Minolta Maxxum cameras since I'm more familiar with this system, but Canon, Nikon, and Pentax autofocus cameras work the same.

My old Maxxum 9000 lacks predictive autofocus but includes focus priority when motor drive is attached. Focus priority locks shutter and readjusts focus until subject is in sharp focus. Focus priority is included in all currently made autofocus cameras as well. When I started using autofocus, I would select focus priority with a fast shutter speed. Camera would focus and re-focus as long as I kept the subject inside the autofocus point and kept the shutter release pressed down. This technique did not ensure sharp focus. Slower autofocus speed and lack of focus tracking resulted in a lot of unsharp images. Subject had to be inside the autofocus point at the exact moment you released the shutter and your shutter speed had to be fast enough in order to take sharp images. Your shutter speed still has to be fast enough with today's camera, except predictive autofocus solves some of the focusing problems. It took me a while to get used to predictive autofocus. My Minolta Maxxum 9 as well as other newer Maxxum, EOS, Nikon AF, and Pentax AF cameras have continuous autofocus that can predict where the subject will be the moment you press the shutter. This is an excellent tool for action photography and does not work like the older models.

First press the shutter release slightly down to activate autofocus. Camera will continue to adjust focus as long as you keep the pressure on the shutter release. Press the shutter release all the way down to take the picture. I have noticed that the subject does not necessarily have to look sharp in the viewfinder while in continuous autofocus mode. Actually, for subjects that are coming towards you for instance, an unsharp image in the viewfinder is a sign of a sharp photograph. With my Maxxum 9000, I had to wait to see a sharp image just as I did with my Minolta X-700 manual focus camera before I released the shutter. With current autofocus cameras, I can just compose the shot and let the camera track and focus on the subject. Predictive autofocus will select the best focusing point for the moving subject and readjust focus as needed. I must admit that some of the shots I have taken of fast moving subjects would have been impossible to take. At least for me.

Left Image: Minolta Maxxum 9, 300mm f-2.8 APO lens, Fuji Provia, Bogen monopod.
Multisegment metering, approx. 1/250 sec at f-2.8 on aperture priority and continuous autofocus.
 
What kind of an action photo is this? If you think this guy was just posing for me, think again. This was one fast active bird (I don't know the specie) who didn't want to sit still for one-second. I spent an hour with this bird and couldn't make him cooperate. He would make fast and strange moves in all directions. While photographing, bird looked blurry in the viewfinder. Surprisingly, autofocus with tracking recorded sharp images of about twenty-five out of thirty-six shots I took, including this one. The bird was moving its head back and forth while turning its body left and right! I have photographed fighter air crafts flying at 600 mph easier than this guy.