What is a fungus?
Most people see a mushroom as a plant, but a close examination shows that there are fundamental differences:
This is why they are included in their own kingdom which in some ways is closer to animals than plants.
The Kingdom Fungi includes some of the most important organisms, both in terms of their ecological and economic roles. By breaking down dead organic material, they continue the cycle of nutrients through ecosystems. In addition, most vascular plants could not grow without the symbiotic fungi, or mycorrhizae , that inhabit their roots and supply essential nutrients. Other fungi provide numerous drugs (such as penicillin and other antibiotics), foods like mushrooms, truffles and morels, and the bubbles in bread, champagne, and beer.
Fungi also cause a number of plant and animal diseases: in humans, ringworm, athlete's foot, and several more serious diseases are caused by fungi. Because fungi are more chemically and genetically similar to animals than other organisms, this makes fungal diseases very difficult to treat. Plant diseases caused by fungi include rusts, smuts, and leaf, root, and stem rots, and may cause severe damage to crops. However, a number of fungi, in particular the yeasts, are important "model organisms" for studying problems in genetics and molecular biology.
Ok, but what is a contex with a lens?
Have you looked through your camera lens recently? Not at the image you see in the viewfinder, but through the actual lens? If not, you might as well do it, especially if your lens is more than a couple of years old. Make sure the aperture is wide open. With some lens mounts this may mean moving a lever, or even mounting the lens on a camera and looking into the front of the lens. If what you see even remotely resembles the image above or seems to be more or les foggy, you may be in a serious trouble.
Does it affect Image Quality?
In its early stages, fungus does not show up on the photos. It is actually pretty dark, and since images of no lens surfaces show up on your picture, you will never get an image like that by accident. The fungus usually starts its life in the baffles and dust in the lens barrel, and no visible effects may be seen for years. Once it reaches out onto the glass surface, it will slowly cover it with a mesh of lines. The first effect is probably very slight loss of image brightness, followed by increased flare from light reflecting off the growth. In its final stages, the lens surface may become etched by the chemicals excreted by the fungus, at which point image sharpness becomes affected. You will probably be able to use the lens for several years before it reaches this stage. Once you get there, you will need to replace the lens.
How to Avoid Fungus?There is no recipe which would give you 100% guarantee that you will never get fungus in your lens. The following suggestions may reduce your risk, however.
Keep your lenses in a dry, cool place. If you use your equipment in damp environments, let it dry as soon as you return into a drier room. Never leave your equipment in a closed (splashproof) camera bag when it need not be there. Leather lens cases are known to be especially bad, because leather is a natural product where fungi can grow before they proceed on to the lens. Synthetic materials are better, but there is no guarantee there either.
If your lens changes its length when focusing or zooming (most lenses do), outside air is sucked in when using the lens. If you come in from moist environment and just leave the lenses lying around, it will take days before the moisture inside the lens drops to ambient levels. You might zoom the lens back and forth several times in a dry room before putting them away, to replace some of the air inside.
If you store the equipment in a sealed (airtight) case, adding a small bag of silica gel or other absorbent may help. You will need to monitor its status (usually a change in color of an additive) regularly. As soon as the gel shows signs of saturation with water, it should be replaced or dried. Otherwise the process will reverse, and instead of drying the case, it will add moisture to the air and make things worse. For the same reason I would recommend against water absorbers in any situation where the container is not sealed airtight.
Several people have asked me about the best relative humidity to keep their lenses at. I have not seen any hard data on this, but anything below 50% should be reasonably safe. If you go below 30%, the lubricants in the lenses (diaphragm, focus, zoom mechanisms) might dry out faster than you like. The best temperature would probably be around 20 degrees C (68F).
What Can I Do to Remove the Fungus?Once it is there, there is very little you can do yourself. You may slow down or stop the growth by following advice from the previous section. I have heard suggestions of using ultraviolet radiation (sun rays) to kill it, but since parts of the growth are outside of the glass area, this is probably only a temporary solution. Some types of glass are not transparent to parts or the whole of ultraviolet spectrum, and the radiation will be absorbed before ever reaching the fungus. Why not try to use poisonous gases, but (1) they may kill you before they kill the the fungus, and (2) they may corrode the mechanics of the lens, so better do not try this.
Taking the lens apart and having it cleaned may be an option. Those who had done it this way have had both successes and failures