We spend a great deal of money on our lenses as this is one of the largest determining factors of image quality. The above picture is of a beloved Olympus 50mm lens I received with my OM-1 camera. The fact is that the equipment had not seen the light of day in years and was stored in a rather humid environment.
When I saw this on the lens, I did not notice it on the images I took, I began to worry about it spreading to other lenses and what I could do to prevent my clean lenses from developing this. I began doing a bit of digging, a great deal of forum reading and some chatting with various photography professionals. Based on all of this I have developed a theory on lens fungus and a method which I believe will help prevent it. This is my own system and I offer no guarantees other than it is a what I use on my lens collection.
Fungus is Bad:
Ok so here is the first controversial thing I will say. While I would hate my lens collection to develop fungus it can give character to images shot with the lens. It can soften or even present a slight haze to the image. Many of the images that I shot with this lens have a soft quality to them which I love.
The problem is that as the fungus grows, lens elements will be permanently damaged and what may start off as a pleasing character may evolve into something undesirable.
Yes and no. Fungus will grow however if a lens is well put together and has not suffered damage, it should not allow any spores into the lens elements. Fungus that typically grows in a lens was present when the lens was assembled. The spores were present and once a bit of moisture condensed into the lens element then it begins to grow.
The most common reason for the growth which I have found is that the fungus is eating organic material on the lens, typically from oils used in construction. In theory if the lens has not suffered damage it should not allow spores into the lens elements. Hence it is not likely for fungus in one lens to spread to another.
How to Avoid:
Based on the above, the objective is to prevent any spores that were present during manufacturing to grow. The trick is to understand what fungus likes...it likes dark, damp places. Exposing the lens to UV light will deter fungus from growing. Keep the lenses in a dry environment and you should be fine.
What I Do:
I take my lenses out around once a month and lay them out on a table in direct sunlight. I am careful to ensure the angle is such that a fire does not start. I then slowly inspect and clean each lens.
I store all my lenses in an air tight box with moisture absorbing silica gel. These are reusable if you heat them in an oven. When I take my lenses for their sun bathing I pop these little things in the oven and dry them.
The whole affair takes me around 2 hours and I enjoy the process. We all like the gear that we buy so spending a bit of time cleaning and maintaining it is not a tall order.