Canon EOS 70DN Basic Instruction Manual page 550

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> or <B> button.
: Metering start
When you press the shutter button halfway, only exposure metering is
performed.
: AE lock/FE lock
Normal shooting (No flash)
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.
With flash
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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