On my Nikon cameras, I use the Continuous Servo AF (AF-C), which corresponds to AI Servo mode on Canon cameras. The reason for selecting this mode is to quickly track performers on stage without having to reactivate AF as they move. For general event photography, I recommend continuous mode.
Before describing why AF-C is the best solution for my shooting, it's important to describe how I have my AF controls set up. By default, most DSLRs have the activation triggered by a half-press of the shutter release.
I have my controls set up so that the activation of AF on my cameras is toggled by the AF-ON button on the back of the camera, thus uncoupling the steps of AF and making the image. In addition, I have AE Lock (exposure lock) set to half-press via the shutter release.
By separating the operations of focus, shutter release, and exposure lock, the photographer gains control over the basic and essential elements of image capture. From obtaining and locking focus, locking exposure, and finally establishing the composition, each step is independent from the others, subdividing the process of capture into a more natural, highly controlled sequence. Furthermore, this hierarchy of use not only grants command over the imaging process, but also simplifies pre-focusing and facilitates quick compositions.
Also, in this setup, AF-C allows for active focus without repeatedly depressing the shutter release button.
When AF activation is tied to the shutter release, using AF-C would necessitate either re-acquiring focus before every image, thereby slowing down composition, or maintaining a half-press to hold focus.
With AF activation dedicated to the the AF-ON button, the photographer is more free to achieve AF lock more independently, allowing for faster and more responsive composition and execution, all with the benefits of active focus acquisition.
For real life event photography, the benefits of this AF setup translate into being able to quickly acquire focus on a subject, maintain focus, and release the shutter in controlled, deliberate steps.
For very quickly moving subjects, I will change the active AF point, selecting the point closest to the target of focus as possible and tracking from there. Other times, various degrees of focusing and recomposing are appropriate.
After acquiring AF, I'm free to turn my attention to composition and release the shutter at the decisive moment, tracking and/or adjusting focus as necessary.
In the instances when the subject moves out of focus, by virtue of extreme movement or the the camera losing its tracking, AF-C helps resume focus as fluidly as possible, while still maintain an awareness of composition and lighting, so that when the lock is resumed, the camera's ready to go.
Using Single Mode
Very rarely, I will switch to Single Servo AF (AF-S) if lighting conditions are very tricky, or when using lenses that behave poorly when used in continuous mode. With regard to the latter, this generally means older, screw-driven AF lenses that do not have built-in motors for changing focus.
One other situation in which I may use AF-S is for relatively static subjects, such as singer-songwriters, when I don't need to worry about quick tracking.
How do you have AF setup on your camera? Do you have any tips to share for tricks to optimize autofocus performance for concert photography?