Yes, even high end view finders could fog up. Tested a few things, and this FOGTECH anti-fog liquid works great and is easy to apply. Just apply it with a wet pad and let it dry for 5 seconds. Makes a huge difference. A good thing to wipe on the GoPro HD Hero as well when working in humidity. Enjoy the tip, found below.

FogTech Anti Fog Tech 30ml Bottle For Paintball Mask