Canon EOS 5D MARK III Instruction Manual page 24

,_,2: Custom Controls
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During flash photography, pressing the button assigned to this function
will fire a prefiash and record the required flash output (FE lock).
You can set the ISO speed by holding down <8> and turning the
<_> dial. If Auto ISO is set, manual ISO speed setting will take effect.
Auto ISO cannot be set. If you use this function in the <M> mode, you
can adjust the exposure with the ISO speed while maintaining the
current shutter speed and aperture.
During metering, you can set the ISO speed by turning the <O> dial.
If Auto ISO is set, manual ISO speed setting will take effect. Auto ISO
cannot be set. If you use this function in the <M> mode, you can adjust
the exposure with the ISO speed while maintaining the current shutter
speed and aperture.
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In manual exposure <M>, you can set the shutter speed with the
<_> or <O> dial.
In manual exposure <M>, you can set the aperture with the <_> or
<_> dial.
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