Originally written in haste, I decided to do some real research and back up my conventional wisdom and correct any stupid statements on my part (not that there would be any, LOL!)
I’m happy to say my original post isn't too far off from this revised version. However, I could not find current corroboration for everything. So if you've encountered THAT version before this version, feel assured that what I wrote there is 'essentially' true however I don’t think I explained some things properly.
What is lens fungus and why is it bad?
Lens fungus can only grow on glass elements that are coated with biological lens coatings. That sort of coatings aren't used that much any more (if at all). Note that I did find some belief that some of the "Rare Earth" type of glass could theoretically contain enough food to allow fungal growth on lenses. However, I suspect that modern manufacturing techniques would probably minimize any significant infestation of fungal spores inside the sealed lens assemblies in the first place.
Other biological contaminants can be used by fungus as food. It is important that other contaminants be kept to a minimum within a lens assembly. This would include dust, lint, oils, varnish, dirt and glue. However, it has been my experience that this stuff is rarely found inside a sealed lens element assembly, and the varnishes and glues used in modern manufacturing techniques tend to be inherently anti fungal in nature. I have seen some dust in some lenses, but in my experience, it doesn't seem to affect the optical quality or fungal growth in modern lenses (lenses, say, less than 20 years old).
And the fungus really can only grow in warm, humid environments anyway. One of the web pages I used as a reference is the Zeiss service info page on fungus on lenses. It claims a humid environment has a relative humidity of 70° and is between 10℃ and 35℃ (50℉-95℉) for three consecutive days. But that would mean most of North America and parts of Europe are a humid environment during the summer, and I simply don’t believe that part of the world generates much in the way of lens fungus. I suspect this is a case of a manufacturer being extra cautious. (But all in all, it’s a good reference page!)
The outside of the front and rear lens elements are so RARELY infected (because they are exposed to outside air of varying degrees of humidity) that it has never been considered much of an issue. Yes, they theoretically can be infected, but I haven't even HEARD of such an infection, much less seen one.
No lens is perfectly sealed, and over time, warm moist humid air seeps in and has difficulty in seeping out quickly enough to stabilize the environment to safe levels of humidity. That, coupled with biological lens coatings makes for an environment suitable for fungus.
Condensation can help with fungus growth, but it isn't a significant factor. The interior of a lens assembly is sealed well enough that the 'dead' air inside acts like an insulation barrier for the internal assembly. Humidity can seep in slowly, but the assembly is sealed well enough to prevent the rapid heating or cooling of internal components that cause condensation. The condensation on EXTERNAL components are irrelevant.
Fungus that grows inside a lens was put there at the time of lens assembly. The spores are too big to pass through the lens assembly seals. Fungus CAN NOT infect your camera body NOR can it pass from one lens to another. ALL older lenses are infected at the time of assembly NOT after assembly.
author’s note: I recall reading the stuff above about condensation and point of contamination years ago (either "Modern Photography" or "Popular Photography" magazines, I believe) , but I can not find any current corroboration. So incorporate this into your personal knowledgebase at your own risk.
How to tell if you have a Fungus infestation
Shine a flashlight into the lens, fungus will appear as soft, white dots scattered about the lens. Under magnification, they will appear to be even smaller clumps connected by delicate tendrils of fungal infection.
How to prevent and remove fungal infections.
Zeiss (here) recommends special climate controlled cabinets heated to between 40℃ - 50℃ (104℉ - 122℉ ). But for the average amateur, that seems like an excessive expense.
I looked for some simple easy ways to help reduce the problem. I make no claims for the value of these suggestions, pleased be advised you alone are responsible for any damage you do to your lenses or other property!
Store the lenses next to silica gel packets, and recharge those packets regularly by heating to the temperatures recommended by their manufacturers.
UV light retards the growth of fungus. In humid climates aim the lens (off camera) at the sun or other uv light source for several hours each day, with the rear lens cap on. However I found two common caveats in my research.
- Don’t aim the sun or other UV light source straight down the barrel of the lens, since theoretically the focal point might well fall on the rear lens cap and melt a hole through it. placing the lens at a slight angle seems to be to most common suggestion.
- Don’t use “blacklight” style lamps the level of UV rays generated by them are not strong enough to do any good.
Cameras can't really get infected since there is little inside a modern camera body that will feed fungus spores, allowing them to grow. The movement of lubricated mechanical parts (that still use biological lubrication - few parts like that, I reckon) would likely grind any fungus to bits before it became a problem.
EDIT 5/28/2015: I did hear from a reader that suggested that the rubberized focal-plane shutter curtain could probably host a fungal infection. which, now that I think about it, seems reasonable. I did some casual internet research on this and I didn't find any supporting information on this though. This may be the cause of a camera itself smelling 'funny'. Based on the results of my previous research on lenses, while there might be some increase in the fungal infection of lenses from the focal plane shutter, I suspect the actual incidence would be VERY small. If you have a camera which you suspect is infected in this way, I would suggest that you store the camera with the lens off and only attach it prior to a shoot.
Modern lenses are assembled in air conditioned and air filtered environments, infection is not likely and subsequent infection is so unlikely (non biological coatings, - on food for fungus, etc) that it just isn't a significant issue.
You don't want fungus on a lens, and don't buy a lens with it. But lens fungus is not the ebola virus it CAN'T spread in any significant way, it can pretty much only ruin ONE lens.