As with many things in life, prevention instead of cure is the preferred path for keeping your lens free of fungal growth. In this case prevention involves keeping your camera gear in a dry, dust-free environment, such as a camera bag, so that airborne fungus spores cannot infect the lens or any other part of your camera. However, if a fungus does take root on the surface of your lens, you should remove the biological organism before the infestation has penetrated the lens coating or lens body.
Remove the lens (if possible) and examine the front and rear elements closely to determine the extent of the fungal growth. If the lens coating has been broken down, the camera and/or lens will have to be taken to a professional to be repaired and re-coated. In most cases this solution is very expensive.
Disassemble the lens, if possible. Only limited cleaning and fungus removal can be performed on lenses that are not interchangeable or come apart easily.
Mix up some hydrogen peroxide and ammonia to soak the infected lens elements. According to the Leica camera company of Germany, the anti-fungal mixture should contain 94 per cent distilled water, 4 per cent ammonia and 2 per cent hydrogen.
Soak the lens for one hour in the ammonia and hydrogen peroxide mixture.
Clean the lens with a special microfiber towel, which is widely available online and at a variety of retailers, including auto supply parts stores.
Place the lens elements in front of a UV light (sunlamp) for 20 minutes in a dust-free environment to further kill any fungi.
Put the lens back together.
Fungi are living biological organisms that live off the chemical breakdown of organic matter. They can be either macroscopic or microscopic in their appearance. Fungal invasion requires the presence of fungal spores and the presence of nutrients and water that the fungus needs for sustenance. Keeping the microscopic spores off your lens is virtually impossible, but not allowing nutrients and water to accumulate on the lens surface is possible. Use a UV or skylight filter to protect the front lens. Vinegar, which can act as a natural fungicide, can also be used to clean the lens.
Do not carry food items in your camera bag. This only increases the chance of a fungal infection. Don't breathe on your lens to clean it during normal use.