Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 242)

O Setting the Desired Exposure Compensation
242
If [ z 2: Auto Lighting Optimizer ] (p.189) is set to any setting other than
[ Disable ], the image may still look bright even if a decreased exposure
compensation for a darker image is set.
You can also set it by pressing the < O > button and turning the < 6 > or
< 5 > dial.
The exposure compensation amount will remain in effect even after you
set the power switch to <
2 >.
After setting the exposure compensation amount, you can set the power
switch to <
R > to prevent the exposure compensation amount from
changing inadvertently.
If the exposure compensation amount exceeds ±3 stops, the end of the
exposure level indicator will display <
J / > or < I / >.