A lot of times you’ll find a lens on Ebay that is selling for cheap because it has “dust” or “mold” inside.
First we’ll consider dust. Usually a little dust doesn’t affect optical quality at all; it takes quite a bit of dust to make a noticeable difference. If you don’t believe me, put a toothpick directly in front of the lens and see if it makes much of a difference. A few specks of dust are inconsequential. But how did the dust get in there in the first place?
Lens dust comes from two sources. In sealed lenses, dust is not from the outside atmosphere, but from little particles that break off of the diaphragm or other mechanisms. Some lenses are more prone to this than others. A lot of dust in a sealed lens also might be a sign that the lens has suffered a lot of mechanical wear.
Most zoom lenses get longer when they are zoomed. Thus, when the lens gets longer, air, along with any dust in the air, gets sucked inside. So if a car speeds past you on a dusty dirt road, don’t zoom the lens!
Most camera shops can hook you up with someone that can disassemble your lens and remove the dust, as well as grease and adjust everything.
Fungus growing inside is a much more serious problem than dust. Growing fungus releases acids which corrode coatings and etch glass. But what food do the fungi eat? It turns out that the glue used to cement lens elements together makes a rather tasty meal for fungi.
Fungus usually results in a “haze” in your photos. But where the fungus is growing is very important. If it’s on the edge of a lens element, and doesn’t affect image quality, then it’s not a huge deal. Fortunately, fungus usually starts growing on the edges of lens elements. If you shoot at a small aperture like f/16 or f/32, then only a small portion of the center of the lens is actually being used. Mold outside of that small region won’t show up in your photos.
Fungus tends to grow when lenses are stored in cool, dark, high-humidity environments for long periods of time. Many people in humid environments store their lenses in air-tight boxes with a de-humidifying material inside (silica gel, etc. You can buy a bucket of it at Home Depot). Get a humidity gauge and aim to keep humidity less than 50%. Less than 30% might dry out the lubricants faster than they would otherwise. It’s kind of like a reverse humidor for your lenses. But hey, if you have $40,000 in lenses, it’s definitely worth the trouble. Another alternative is to store your lenses in a box with a light bulb inside.
Another good way to keep mold from growing inside your lens is to simply use your equipment. Sunlight is great at killing mold, since mold can’t grow in light. Nothing hurts a camera and lens more than storage. If you aren’t shooting for extended periods of time, take your lens, remove the caps, and place it in a bright window from time to time.
In the field, you can take small desiccant packs and put them in your camera bag to decrease the humidity inside. And remember, the WORST thing you can do is leave your lenses and cameras in a slightly damp camera bag overnight.
So if you have fungus in your lens, how do you remove it? You can slow down the growth by putting the lens in light, lowering the humidity, etc. A professional lens cleaning service will probably cost $50 to $100 depending on the lens (you might be able to buy an identical used lens instead). And residual fungus will probably remain in the lens. And this won’t help you a while lot if the glass has already been etched by the fungus.