Here is my answer to this member's question:
A: Dear eBay Member,
Fungus is caused by a combination of moisture and dust (containing fungus spores) inside the lens. Some dust gets inside every lens that isn't fully "weather sealed" (like an expensive "L" professional model).
When moisture gets inside your lens, either because of direct moisture exposure or simply because of very high humidity in your local climate, fungus can begin to grow. Some lenses that I have examined from very humid "tropical" areas have had the worst cases of fungus growth that I have seen. (Not all of them, of course, but there is a slightly higher percentage of lenses from these areas that have been affected.)
A very unusual problem is caused when lenses are exposed to extreme cold and then brought into a warm building. The moisture inside the cold lens condenses into liquid water or "fog" on cold interior lens surfaces, especially when it is allowed to warm too quickly. The best way to control this is to leave the lens in its closed protective case and allow it warm very slowly after is brought inside from sub-freezing conditions.
Overall, the best way to control moisture is to keep small packets of "Silica Gel" desiccant inside the case with each lens. (Most new lenses come with a Silica Gel packet inside the box. Keep it and use it!)
When moisture finds its way inside the lens, it provides a rich medium for the fungus spores that are often mixed in with dust particles (remember, they were already inside the lens). The fungus then begins to grow and it multiplies in the moist environment. If this problem is recognized quickly, a professional lens technician can clean it. If it is allowed to multiply over time, it causes permanent "etching" in the lens coatings. This condition is not usually reversible and often permanently affects the image quality of the lens.
The preferable solution is PREVENTION. However, if you find the signs of fungus when you look through your lens, have it inspected by a professional at as soon as possible. Hopefully, you can have it cleaned before the problem becomes more serious.
Thanks for your very interesting question!
Introduction To The Q & A Guide Series:
I have been answering potential bidder's questions about the items that I have listed on eBay for several years. One kind person suggested that there may be other members who have wondered about the same subject, but may have been too shy to contact me directly. He suggested that it could be useful to post his question, and my answer, for others to see. He recommended that it could be done in the form of an eBay Guide.
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