Canon EOS Rebel T6i EF-S 18-55mm IS STM Lens Kit Instruction Manual - Page 101

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.  If focus cannot be achieved, the focus indicator in the viewfinder will blink. If this occurs, the picture cannot be taken even if the shutter button is pressed completely. Recompose the shot or see "Subjects Difficult to Focus on" (p.110) and try to focus again.  If [z1: Beep] is set to [Disable], the beeper will not sound when focus is achieved.  After achieving focus with One-Shot AF, you can lock the focus on a subject and recompose the shot. This is called "focus lock". This is convenient when you want to focus on a peripheral subject not covered by the Area AF frame.  When a lens equipped with electronic manual focusing function is used, after achieving focus, you can focus manually by turning the lens focusing ring while pressing the shutter button halfway. 101