Canon EOS-1DS Mark III Instruction Manual page 162

Custom Function Settings
While holding down the <_> (AE lock) button, you can switch to the
registered setting (shooting mode, metering mode, shutter speed, aperture,
or exposure compensation).
Disable: Pressing the <_> button wilt lock the exposure (AE lock).
Enable: By holding down the <-)(-> button, you can instantly switch to the
registered setting.
Register: Set the desired setting for the AE lock button: shooting mode,
metering mode, shutter speed, aperture, or exposure
compensation.
When you select [Register], [With AE !ock button (AF on/AF
off}] will be displayed where you can set the AE lock button to
also execute AF or not. Select [AF on/AF off] to register the
setting to the camera.
0: Auto
1:1/250 sec. (fixed)
Sets the flash sync speed to 1/250 sec. in the aperture-priority AE (Av)
mode. (Against dark backgrounds such as the night sky, the subject's
background will look dark.)
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