Has it ever happened to you that you were taking photographs on a cold day, and when you came into the house, your camera condensed?
I think this happened to all of us. Or at least to all of us leaving in colder climate.
What happens when you get indoors after being outside on a cold day is the moisture is being formed inside the camera and it creates fog-effect inside the lens and often inside the viewfinder as well.
What this may cause to your camera is potential damage as well as the moisture condensing inside the camera is not good for the electronics inside the camera. Fog that creates on your lens, which is more important to worry about than fog on the viewfinder, may even make your camera ‘out of order’ for quite some time. I know this happened to me in the past, in winter.
What you need to do is avoid quick changes in temperature, especially changes from warm to cold.