How to stop your GoPro lens from fogging up
Most of us have had this happen: We charge up our GoPro, get out to the location of whatever it is we're doing that day, do something incredible with the GoPro recording, then get back home and see that the lens was fogged up and the footage is unusable. It sucks.
If you are using the dive case, then you could just put an anti-fog insert in the dive case, but what if you're not?
Why makes a GoPro lens fog up?
One of the major selling points of a GoPro is that it's waterproof - but that also causes us some trouble. If we open the GoPro by taking off the lens cover or changing the battery in an environment with any humidity at all, we are trapping water inside the camera or behind the lens cover.
Then when we go outside when it's cold (or go under water), the cold air hitting the lens cover causes a temperature differential between the outside of the camera and the inside of the camera. This temperature differential causes the water inside the camera (because you changed the battery in a humid environment!) to condense on the lens cover, causing fog. It's a bit like what happens when you get in your car on a cold day and your windshield fogs up.
Most modern GoPros actually are actually waterproof even without the lens cover on. This is because the actual (internal) lens inside the camera is sealed, meaning that the space between the glass lens cover and the internal lens is sealed off from the rest of the camera. What this means is that there are actually 2 places where moisture can be trapped: 1) inside the camera itself; and 2) in the small air space between the glass lens cover and the actual lens in the camera (this is the most common place to fog up).
Stop the fog in the first place
There are two components to what makes a lens fog up - the humid air in the camera and the temperature differential. Let's look at these separately:
Getting rid of humidity in the camera
The best way to de-humidify your camera is to take out the battery leaving the battery door open and remove the lens cover if it's removable. Then put it in a sealed plastic bag with a desiccant package - you know, those silica gel packs that have "DO NOT EAT" written on them. Leave it for an hour or so (overnight is better if you can), then put the lens on and the battery back in, and close the battery door (all still inside the plastic bag if you can). It is not absolutely necessary to dry out the lens cover (since it doesn't really hold any air), but it can help just in case there is moisture on it.
If your camera does not have a removable lens cover (i.e. Hero 8 and Session), then just open the battery door.
If you don't have any desiccant packs lying around, you could put the camera in the fridge for 30 minutes or so - fridges dry out the air due to how they are cooled (which is why your veggies wilt quickly unless they are kept in a crisper drawer). Put your camera back together in the fridge where the air is dry.
One last method is to put the camera close to a heater while opened. The heater will dry out the air, but just be careful not to have the heat blasting directly at the camera because the heat is not great for the electronics.
Once you've dried all the air in your camera, make sure to never open your camera (ie to change the battery) in a humid environment. If you are out and about, you could do it in an air conditioned car. It's also a good idea keep your plastic bag with the silica gel pack around to store your camera when you're not using it. Just remember to replace the silica gel pack once in a while, they can become saturated and start to leak messy stuff.
Temperature differential
If we could keep the camera internals at exactly the same temperature as the outside, then we wouldn't have as much of an issue with fogging. Solving this can be challenging, especially because GoPros generally heat up when recording.
The best thing that you can do is to make sure that your GoPro is acclimatized to the cold before using it. That means don't take your GoPro from a warm car into the cold and start using it right away. Leave it outside (or in cold water if you're going to be using it underwater) in the shade (not in the sun!) for about 30 minutes to cool off before using it.
The other thing that you can do is to make sure that your camera is actually powered down after you stop recording to keep heat to a minimum. You can also turn off voice control, Wi-Fi and GPS while recording to keep the camera cooler.
Get rid of fog once it's there
OK, in spite of everything, you've got a foggy lens. What can be done to fix it? First, do not try to clean the inside of the lens cover! it's easy to get streaking on the glass and very difficult to remove. You're going to have to let the moisture go away on its own.
Take your GoPro, remove the lens cover, open the battery door and let it sit in a dry environment like an air conditioned car or your plastic bag with the desiccant pack.
Perhaps counter-intuitively, instead of using an air conditioned car you could alternatively put the parts in front of a car heater to dry them out, but you will most likely need to acclimatize your camera again because it will be warmer than it is outside.