C.fn Iv: Operation/Others; Shutter Button/Af-On Button; Af-On/Ae Lock Button Switch - Canon EOS 50D Instruction Manual

It is best to make the adjustment at the actual place to be photographed.
This will make the adjustment more precise.
With setting 2, if an Extender is used, the adjustment will be registered
for the lens and Extender combination.
The registered AF microadjustments will be retained even if you use the Custom
Function to clear all settings (p.172). However, the setting itself will be [0: Disable].

C.Fn IV: Operation/Others

C.Fn IV -1
0: Metering + AF start
1: Metering + AF start/AF stop
During autofocusing, you can press the <p> button to stop the
autofocusing.
2: Metering start/Metering + AF start
This is useful for subjects which keep moving and stopping repeatedly.
In the AI Servo AF mode, you can press the <p> button to start or
stop the AI Servo AF operation. The exposure is set at the moment the
picture is taken. Thus, the optimum focusing and exposure will always
be achieved as you wait for the decisive moment.
3: AE lock/Metering + AF start
Convenient when you want to focus and meter at different parts of the
picture. Press the <p> button to meter and autofocus, and press
the shutter button halfway to attain AE lock.
4: Metering + AF start/Disable
The <p> button will not function.
C.Fn IV -2
0: Disable
1: Enable
The functions of the <p> and <A/y> buttons will be switched
with each other's function.
When set to 1, press the <p> button to display the image index or to
reduce the image display.

Shutter button/AF-ON button

AF-ON/AE lock button switch

3 Custom Function SettingsN
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