Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 403)

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11
Sensor Cleaning
The camera has a Self Cleaning Sensor Unit to
automatically shake off dust adhered to the image
sensor’s front layer (low pass filter).
The Dust Delete Data can also be appended to the
image so that the dust spots remaining can be deleted
automatically by Digital Photo Professional (EOS
software, p.552).
Smudges adhering to the front of the sensor
Besides dust entering the camera from outside, in rare cases,
lubricant from the camera’s internal parts may adhere to the front of
the sensor. If visible spots still remain after the automatic sensor
cleaning, having the sensor cleaned by a Canon Service Center is
recommended.