Canon EOS 6D Instruction Manual page 324

8: Custom Controls
: Metering and AF start
When you press the button assigned to this function, metering and AF
are executed.
: AF stop
The AF will stop while you hold down the button assigned to this function.
Convenient when you want to lock the focus during AI Servo AF.
: ONE SHOT z AI SERVO
You can switch the AF operation. In One-Shot AF mode, when you hold
down the button to which this function is assigned, the camera switches
to AI Servo AF mode. In the AI Servo AF mode, the camera switches to
One-Shot AF mode only while you hold down the button. Convenient
when you need to keep switching between One-Shot AF and AI Servo
AF for a subject that keeps moving and stopping.
: AF point direct selection
During metering, you can select an AF point directly with <9> without
pressing the <S> button.
: Metering start
When you press the shutter button halfway, only exposure metering is
performed.
: AE lock/FE lock
AE lock: When you press the button assigned to this function, you can
lock the exposure (AE lock) during the metering. Convenient
when you want to focus and meter the shot at different areas
or when you want to take multiple shots at the same exposure
setting.
FE lock: During flash photography, pressing the button assigned to this
function will fire a preflash and record the required flash output
(FE lock).
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