Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 432)

3 Custom Function Settings
432
FE Microadjustment C.Fn3
You can fine adjust the camera’s standard flash exposure level. The
adjustment can help if the camera’s “standard flash exposure level”
(without flash exposure compensation) always seems to underexpose
or overexpose the subject.
OFF: Disable
ON: Enable
Select [ Enable ], then press the < Q > button. The adjustment
screen will appear. The adjustment can be made up to ±1 stop in
1/8-stop increments. If the flash exposure level tends to
underexpose the subject, set it to the plus side. If it tends to
overexpose, set it to the minus side.
Normally, this adjustment is not required. Perform this adjustment
only if necessary. Note that doing this adjustment may prevent
obtaining the correct flash exposure.