Understanding lens fungus:Lens fungus is a microbial growth on glass surfaces of camera lenses and other optical devices. It is literally a disease of lenses which is very common. If you own lenses but haven't seen it, it is probably because you haven't looked. It takes a number of forms but the typical destructive growth pattern from microscopic spores growing in webbed mycelium patterns across an optical surface. In some cases this can be cleaned off and the lens restored, but often the metabolic products of the fungus etch the glass lens surface or coatings, or the growth may be on internal glass surfaces that cannot be reached for cleaning. Like dust or fingerprints on a lens, obstruction and obscuration of the lens progress in degrees, from an slight loss of image contrast and resolution to complete ruin.

Using a flashlight and magnification to identify lens fungus: Effective diagnosis of the presence of lens fungus requires proper lighting and magnification, like any task in microscopy. A casual look through a lens towards a diffuse light source with the naked eye will not reveal any but the most severe cases. The most convenient critical method is to shine a bright flashlight through the lens while looking off-axis at the surfaces under the magnification of a 10X eye loupe. This is a low-power version of what a microscopist would call darkfield illumination. My best tools for this purpose have been simply a handheld 3-watt LED mini-Maglite and a viewfinder eyepiece taken from an old discarded camera. I shine the light through one end of the lens, and hold the loupe to my eye and bring the lens up to a focused magnification. Every lens always has some dust contamination (short of clean-room conditions) that this examination will reveal, but the presence of the spreading fibrils of mycelium look quite distinct from ordinary dust. Fogging and other diffuse imperfections and degradations in the lenses and coating will also be revealed.

Why does lens fungus grow? Fungal spores are everywhere and will grow whenever temperature, humidity, and nutrients exist. Ordinary house dust consists of carbohydrate (cellulose lint from fabrics) or protein (bits of you) that is fine food for fungus. Liquid water and light are not necessary. This dust typically gets pushed into the corners of a lens (ironically) from cleaning, where it collects, and these marginal heaps also tend to retain humidity. Humidity above 60 percent is reputed to be the threshold where fungal growth starts. All it takes is one spore to germinate on a single tiny lint fiber, and out go the fibrils across the glass, looking for more food. A fungal spore can feast on a tiny speck of dust for many days, covering many square millimeters of area with fibrils that will find other food present. Live fungus is basically a molecular machine that is powered by the minute chemical energy available in the dust, which dust is also digested into the substance of the fungal structure. This growth mechanism converts bits of energy-rich food into less energetic but more connected and purposed structures. The structures send forth runners (mycelium fibrils) to locate and tap more energy sources, in hopes of finding a food bonanza that will supply enough energy and material to launch baby funguses (spores) into the air, where they can travel to new, distant food sources. While the lens growth is typically not rich enough to fruit and launch spores, the resulting mycelium will degrade the lens. Besides the mechanical obstruction of the light rays the lens is designed to refract and transmit, the mycelium growth produces chemical byproducts which deter overdense growth, digest its food, and also happen to etch glass.

What can I do to avoid or stop lens fungus? Take away warmth, humidity, or the food source. Since this is not typically possible with perfect continuity, lens fungus is simply a constant threat. Sterilization is not typically practical and doesn't remove existing growth or prevent new growth on re-exposure.