Canon EOS 750D Instruction Manual page 25

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
remain on the image, having the sensor cleaned by a Canon Service Center is
recommended.
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.
Handling Precautions
Contacts
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