Canon MARK IV User Manual (en)

Sensor Cleaning
The camera has a Self Cleaning Sensor Unit attached 
to the image sensor’s front layer (low-pass filter) to 
shake off dust automatically. 
The Dust Delete Data can also be appended to the 
image so that the dust spots remaining can be erased 
automatically by Digital Photo Professional (provided 
About smear 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. 
In case visible spots still remain after the automatic sensor cleaning, 
having the sensor cleaned by a Canon Service Center is recommended.
Even while the Self Cleaning Sensor Unit is operating, you can 
press the shutter button halfway to interrupt the cleaning and start 
shooting immediately.