Canon EOS-1D X Mark III EOS-1DX Mark III Advanced User Guide - Page 354

3. Focus and take the picture. Aim the AF point over the subject and press the shutter button halfway. When focus is achieved, the AF point will turn green and the beeper will sound. If the camera cannot focus, the AF point or Zone AF frame turns orange. Caution The camera continues to switch the active AF point < > to track subjects when Zone AF or Large Zone AF (vertical or horizontal) is set to Servo AF, but tracking may not be possible under some shooting conditions, such as when subjects are small. Focusing may be difficult when using a peripheral AF point. In this case, select an AF point in the center. Tapping the screen to focus will focus with [One-Shot AF], regardless of the AF operation setting. Note With [ : Orientation linked AF point], you can set separate AF points for vertical and horizontal shooting ( ). 354