Canon EOS 5Ds Instruction Manual page 407

3 Custom Function SettingsN
This function does not work with macro lenses whose actual aperture
f/number changes when the magnification changes.
This function does not work with movies.
If [ISO speed] is set and the exposure cannot be maintained within the
range set with [ISO speed range], the ISO speed will be automatically
switched within the specified range.
If [Shutter speed] is set and the exposure cannot be maintained within
the range set with [82: Set shutter speed range], the shutter speed
will be automatically switched within the specified range.
If you do 1, 2, or 3 and the camera turns off (power switch is set to
<2>, etc.) while the exposure is being maintained, the standard
exposure will be updated to the exposure at the moment the camera
turns off.
This function also works with changes in the highest f/number (minimum
aperture).
If you set [ISO speed] or [Shutter speed], do 1, 2, or 3, and then undo 1,
2, or 3 without manually changing the ISO speed, shutter speed, or
aperture, so that the camera is back to its original state, the original
exposure setting will be restored.
If [ISO speed] is set and the ISO speed increases to an expanded ISO
speed, the shutter speed may change to maintain the exposure.
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