Tamron SP 70-200mm f/2.8 Di VC USD G2 Lens for F - Manual (Page 26)

The front surface of the lens element is coated with a protective fluorine compound that is water-
and oil-repellant. The lens surface is easier to wipe clean and less vulnerable to the damaging
effects of dirt, dust, moisture and fingerprints.
26
Fluorine Coating
Image illustrates resistance to grime (oil-based felt marker)
Left side:
Without Fluorine Coating Right side:
With Fluorine Coating