Canon EOS-1D X Mark III EOS-1DX Mark III Advanced User Guide - Page 23

Lens After detaching the lens from the camera, put down the lens with the rear end up and attach the rear lens cap to avoid scratching the lens surface and electrical contacts (1). Smudges on the image 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 smudges are visible on images, have the sensor cleaned by a nearest Canon Service Center. 23