Fungus is a silent destroyer of camera lenses. It thrives in warm and humid conditions such as camera cases. I recently purchased a camera and lens which externally appears in near mint condition. However, fungus has been growing in the lens, covering several lens surfaces and effectively destroying it. Repair would require disassembling the internal components and cleaning, but re-assembly needs calibration equipment, manuals and tools which are no longer available. Even if they were, the cost would exceed the value of the lens on the used market, making repair uneconomical.

That's why it's important to store your camera equipment to prevent fungal growth. It only requires a humidity controlled environment, the most economical solution is to use cheap airtight storage containers and a desiccant like silica gel to absorb moisture. Inexpensive humidity indicator strips show when the desiccant is saturated and needs changing or refreshing.

I use orange indicating silica gel which changes to green when saturated.

This video will show you how to refresh silica gel so it can be reused many times. The process only takes a few minutes, and involves heating the silica gel in a microwave oven for two to three minutes to release the water. I then heat the crystals in an electric oven at 120 to 150 Celcius for 5 to 10 minutes to evaporate all moisture from the crystals. Once cooled don the desiccant are bagged and returned to their containers.