Canon EOS 20D Instruction Manual page 146

Custom Function Settings*
0: AF stop
1: AFstart
AF operates only while the AF stop button is pressed. While the
button is pressed, AF operation with the camera is disabled.
2: AE Iockwhile metering
When the button is pressed while metering is still active, AE lock is
applied, Convenient when you want to focus and meter separately,
3: AF point: M -> Auto / Auto -> ctr.
In the manual AF point selection mode, holding down the button
switches to automatic AF point selection, Holding down the button
switches from manual AF point selection to automatic AF point
selection instantly, Convenient when you are no longer able to
focus track a moving subject with a manually-selected AF point in
the AI Servo AF mode, In the automatic AF point selection mode,
holding down the button selects the center AF point instantly,
4: ONE SHOT <-> AI SERVO
In the One-Shot AF mode, the button switches to the AI Servo AF
mode only while you hold it down. In the AI Servo AF mode, the button
switches to the One-Shot AF mode only while you hold it down.
Convenient when you need to keep switching between One-Shot AF
and AI Servo AF for a subject which keeps moving and stopping.
5: IS start
With the lens' IS switch already ON, the Image Stabilizer operates
only while you press the button,
[] The AF Stop buttonis provided on!Yon super telephotolenses,
0: off
1: On
Data for verifying whether the image is original or not is appended
to the image. When an image appended with the verification data
is played back, the <_1> icon will be displayed. (p.104)To verify
whether the image is original, the Data Verification Kit DVK-E2
(optional) is required.
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