Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 407)

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Dust that could not be removed by the automatic sensor cleaning can
be removed manually with a commercially-available blower, etc. Before
cleaning the sensor, detach the lens from the camera.
The image sensor is extremely delicate. If the sensor needs to be
cleaned directly, having it done by a Canon Service Center is
recommended.
1
Select [Sensor cleaning].
Under the [ 5 3 ] tab, select [ Sensor
cleaning ], then press < 0 >.
2
Select [Clean manually].
3
Select [OK].
X In a moment, the reflex mirror will lock
up and the shutter will open.
“ CLn ” will blink on the top LCD panel.
4
Clean the sensor.
5
End the cleaning.
Set the power switch to < 2 >.
3
Manual Sensor Cleaning
If you will clean the sensor manually, make sure to use a fully-charged
battery.
It is recommended to use household power outlet accessories (sold
separately, p.485).