text: AI Servo AI Servo is an autofocus mode found on Canon SLR cameras, and in other brands such as Nikon, Sony, and Pentax, under the name "continuous focus" (AF-C).  Also referred to as focus tracking, it is used to track a subject as it moves around the frame, or toward and away from the camera.  When in use, the lens will constantly maintain its focus on the subject, hence it is commonly used for sports and action photography.  AI refers to artificial intelligence: algorithms that constantly predict where a subject is about to be based on its speed and acceleration data from the autofocus sensor.
  Focus motors
  Modern autofocus is done through one of two mechanisms; either a motor in the camera body and gears in the lens ("screw drive") or through electronic transmission of the drive instruction through contacts in the mount plate to a motor in the lens. Lens-based motors can be of a number of different types, but are often ultrasonic motors or stepper motors.
  Some camera bodies, including all Canon EOS bodies and the more budget-oriented among Nikon's DX models, do not include an autofocus motor and therefore cannot autofocus with lenses that lack an inbuilt motor. Some lenses, such as Pentax' DA* designated models, although normally using an inbuilt motor, can fall back to screwdrive operation when the camera body does not support the necessary contact pins.