Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 389)

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You can adjust the brightness of the LCD monitor to make it easier to
view.
1
Select [LCD brightness].
Under the [ 5 1 ] tab, select [ LCD
brightness ], then press < 0 >.
2
Adjust the brightness.
While referring to the gray chart, turn
the <
5 > dial, then press < 0 >.
Changing Image Playback Settings
3 Adjusting the LCD Monitor Brightness
To check the image’s exposure, referring to the histogram is
recommended (p.351).
During playback, pressing the < U > button will display the screen in step
2.