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Something I read on the Internet and magazines (for example here) from time to time is that fingerprints and smudge can "ripen" if left on a lens for a long time.

I find this a bit hard to believe and even if there is truth to it there are so many different lens coatings and materials used in lenses that it seems unbelievable that it applies to them all.

Is there any real danger of a having fingerprints left on a lens for an extended time and can they actually "ripen" onto the surface?

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The residue left behind when your fingers touch the glass of your lens contains several organic and chemical compounds.

  • Normal skin oil has a pH level of around 4 to 5.5, which is mildly acidic. The longer it is allowed to be in contact with materials reactive with acids, the more reaction will take place. And yes, acid will eat through the coatings on lenses.
  • Sweat contains salts that are corrosive. As with most corrosive agents, the longer they are allowed to contact things that can corrode, the more corrosion will occur.
  • The organic compounds in skin oil can feed microbes, such as fungus, that might be present on your lens. The longer the fungus is allowed to feed on the organics, the worse your fungus problem will be. Fungus will not only damage a lens' coatings, but will eventually etch the surface of the glass itself.
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    \$\begingroup\$ This seems like the start of an answer.... \$\endgroup\$
    – mattdm
    Commented Jun 29, 2014 at 18:12
  • \$\begingroup\$ Add in the fact that the fingerprint causes additional soils to stick better to the lens, and a "yes it does get worse over time" and yes it can damage the coating, and this would be the answer. \$\endgroup\$
    – Jasmine
    Commented Jun 30, 2014 at 16:25
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    \$\begingroup\$ Sorry, I assumed it to be self evident that mild acids, light corrosives, and organic compounds that feed fungus would increase the damage the longer they are allowed to remain in contact with the lens. \$\endgroup\$
    – Michael C
    Commented Jul 1, 2014 at 2:25

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