Canon EOS 5D Mark III Instruction 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 
software, p.394).
About 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 
Even while the Self Cleaning Sensor Unit is operating, you can 
press the shutter button halfway to interrupt the cleaning and start 
shooting immediately.