Canon EOS 7D - Manual (Page 90)

90
f : Selecting the AF ModeN
AI Focus AF switches the AF mode from One-Shot AF to AI Servo
AF automatically if the still subject starts moving.
After the subject is focused in the One-Shot AF mode, if the subject
starts moving, the camera will detect the movement and change the
AF mode automatically to AI Servo AF.
Under low-light conditions, when you press the shutter button halfway,
the built-in flash fires a brief burst of flashes. It illuminates the subject to
enable easier autofocusing.
AI Focus AF for Automatic Switching of AF Mode
AF-Assist Beam with the Built-in Flash
When focus is achieved in the AI Focus AF mode with the Servo mode
active, the beeper will sound softly. However, the focus confirmation light
in the viewfinder will not light.
The built-in flash’s AF-assist beam is effective up to about 4 meters/13.1
feet.
In the d/s/f/a/F shooting modes, press the button to pop-
up the built-in flash. Then it will fire the AF-assist beam when necessary.