Canon EOS 5D Mark III Instruction Manual (en)

82: Custom Controls
During metering, when you press the button assigned to this function, 
the camera will switch to the AF point registered on page 330.
*3: In step 3 on page 327, when you press the <
B> button, you can select [Switch 
only when btn is held] or [Switch each time btn is pressed].
During metering, you can select an AF point directly with the <5> dial or 
9> without pressing the <S> button. With the <5> dial, you can select a 
left or right AF point. (With Zone AF, the selected zone will change in a loop.)
*4: If you use <
9> and press the <B> button in step 3 on page 327, you can press 
9> straight down to select [Switch to center AF point] or [Switch to registered 
AF point].
When you press the shutter button halfway, only exposure metering is 
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.
The exposure will be locked (AE lock) while you press the shutter 
: Switch to registered AF point
: AF point direct selection
: Metering start
: AE lock
: AE lock (while button pressed)
If you assign [AE lock (while button pressed)] to the shutter button, any 
buttons assigned to [AE lock] or [AE lock (hold)] will also work as [AE lock 
(while button pressed)
If you change the aperture in <a> mode when [AF point direct selection
is set, turn the <6> dial while holding down the <
> button.