Canon EOS Rebel T6s EF-S 18-135mm IS STM Lens Kit Instruction Manual - Page 99

f: Changing the Autofocus OperationK One-Shot AF for Still Subjects Suited for still subjects. When you press the shutter button halfway, the camera will focus only once.  When focus is achieved, the AF point that achieved focus will be displayed, and the focus indicator in the viewfinder will also light up. AF point Focus indicator  With evaluative metering, the exposure setting will be set at the same time focus is achieved.  While you hold down the shutter button halfway, the focus will be locked. You can then recompose the shot if desired. AI Servo AF for Moving Subjects This AF operation is suited for moving subjects when the focusing distance keeps changing. While you hold down the shutter button halfway, the subject will be focused on continuously.  The exposure is set at the moment the picture is taken.  When the AF area selection mode is set to 19-point automatic selection AF (p.101), the camera first uses the manually-selected AF point to focus. During autofocusing, if the subject moves away from the manually-selected AF point, focus tracking continues as long as the subject is covered by the Area AF frame. AI Focus AF for Switching the AF Operation Automatically AI Focus AF switches the AF operation from One-Shot AF to AI Servo AF automatically if a still subject starts moving.  After the subject is focused on in One-Shot AF, if the subject starts moving, the camera will detect the movement, change the AF operation automatically to AI Servo AF, and start tracking the moving subject. 99