EOS 5DS, EOS 5DS R Basic Instruction Manual

f: Selecting the AF OperationN
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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 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.
While you press the shutter button
halfway and the camera focuses with AF,
the <e> icon will appear under the
focus indicator in the viewfinder.
In One-Shot AF mode, the icon also appears if you press the shutter
button halfway after focus is achieved.
By default, the AF points light up in red in low-light conditions.
AI Focus AF for Switching the AF Operation Automatically
AF Operation Indicator
AF Points Lighting Up in Red