Digital Cameras: What is an AF Servo?
Some digital cameras have a function called AF Servo. This is an auto focus function that allows a photographer to keep a moving object automatically focused based on the camera's setting. This function is normally found on professional SLR cameras only. It is commonly used in sports and wildlife photography. This has more advantages than pre focusing because you can follow a moving object while keeping it in focus, as opposed to guessing where it will go.
Understanding AF Servo
AF Servo is usually engaged when you switch to "AI Servo" in a Cannon camera or "Continuous" in a Nikon. Once this is engaged, you find your subject and hold the shutter half way down (to keep him in auto focus as he moves based upon the settings of your camera). You can also fully hold the shutter down and the camera will auto focus while shooting. When doing this, your camera will enter "release priority mode" and it will take pictures when the shutter is down regardless of if the image is in focus or not.
The usefulness of AF Servo is not just limited to sports or wildlife photography. It can come in handy during live events, like a concert, where the subject is constantly moving.