Although rust is the most common problem, it is quite rare. Most often, moisture will affect your binoculars by causing slowed joints or gunk in them. This can make it difficult to focus or adjust the zoom. You can easily treat this by brushing your binoculars' crevices with any type of oil such as WD-40, gun oil or other mechanical oil. Extreme cases can lead to moisture getting into the lenses of your binoculars, which could cause a problem with a component or even a complete loss. Most good binoculars come with seals that prevent water or condensation getting into the lenses. It can also be an issue with cheaper binoculars. Binocs can be killed by condensation within the line of sight. You might temporarily eliminate it by keeping your binoculars dry, but it'll return the next time it rains or is humid.