Let’s take a look at why lenses tend to fog in cold weather. This is something that we constantly teach on our winter workshops during the winter months.
When the air near the lens is cooled by the lens, the relative humidity of the surrounding air will increase. This is because when air cools, it loses its ability to carry water vapor. Once the relative humidity increases enough, water will begin to collect on the lens. Even if the air is fairly dry, when the temperature difference is great enough you will get lens fog.
So how do we stop this? One way is to use hand warmers and place them on the barrel of the lens. This will warm up and dry both the external glass and the internal elements, help staving off mold that could grow inside your lens.
Another method we like, is to use your coat that you wore outside. The outside of the coat is the same temperature as the lens and camera. Take that coat and wrap your camera so the outer shell is against the camera. This allows the camera and lens to warm naturally, surrounded by air that is initially cool—both the air and camera gear will warm naturally, preventing the moisture and fog from forming on your camera gear.
If you have any further comments, please add them to the comments section below. The more feedback and ideas the better.