I wish I knew. I’ve used every mode and combination on the Nikon D70, D200, and D300, and there is no perfect solution for erratically moving subjects that often cross paths with other erratically moving subjects.
I have settled on a basic approach. Generally, I use an upper middle focus sensor and try to keep it on a dancer’s face. If the dance is a little more structured in its use of stage space, I’ll choose a sensor that I can keep over the face of the dancer in the foreground or the best light. If there are only one or two dancers on stage, I’ve had surprisingly good luck with Nikon’s completely automatic setting (the big white rectangle, as described by Ken Rockwell).
For all of my shooting, I use the af-on focus method. I’m not sure what the equivalent is on other cameras, but I do not focus with the shutter release button. Instead, I use a button on the back of the camera, positioned at my right thumb. Using this method, I can hold the button down and get continuous focus as the dancer moves. But if the dancer stops to hold a pose, I can release the button, locking focus, and then recompose before squeezing the shutter button. It takes some practice.
I have also experimented with something called Focus Tracking with Lock-On, which is menu item A4 on the Nikon D300. Focus Tracking with Lock-On determines how long the focus will stay locked when there is a sudden change in the subject’s distance. This is tricky. If you don’t want the camera to instantly refocus when another dancer passes in front of the person you’re focused on, you choose a longer lock time. But then the camera will not react instantly when the individual on whom you are focused suddenly moves forward or backwards. I’m getting a headache just thinking about it. I keep Focus Lock on the “low” setting and pretend I never heard of it. If I ever get to shoot individual dancers, I’ll turn it off (allowing the autofocus system to respond immediately to sudden changes) and see if the keeper rate increases.
When I review a thousand images from a dance concert, there are two immediate piles of rejects: the severely underexposed shots and the out of focus shots. There are usually a few underexposed shots from moments where the lights dropped low but I kept shooting anyway. But there are hundreds of out of focus shots. If any of you know better focusing techniques for dance, particularly with lots of people onstage, please share them in the comments section. Thanks, Z