The last time I was out doing photography in -20 °C weather on our country property, my camera and lens were suddenly coated in condensation when I came into the warm house to get a drink. When I went back out, some of that had not cleared, and the moisture on the equipment froze. How do I prevent this problem in future?
That can certainly be frustrating, Terry. As you discovered, condensation on the camera’s eyepiece, the lens elements and perhaps even the internal reflex mirror or imaging sensor can easily freeze when you change temperatures. (Even “weather-sealed” cameras can have these condensation problems!) However, the method for preventing this problem is straightforward. When entering a warm/humid location, leave the cold photo equipment inside your camera bag with the zippers tightly closed. If you were using a camera with a small lens, the alternative would be to pop the equipment into a Ziploc freezer bag and seal it before entering the warm location. Either tactic will allow the equipment to warm gradually and prevent condensation from forming. (The warming process is definitely quicker in a Ziploc bag.)
If you forget to use one of these strategies and remove the equipment while it’s still cold, condensation will form. In that case, use a hair drier—set to Low heat— to gently but quickly warm and dry the camera/lens. (In a public restroom, the hand drier blower can also work well but keep the equipment a metre or so from the source of the hot air.) Do not remove the lens and blow air into the camera’s internal chamber since that can introduce dust or moisture that may end up on the imaging sensor.
Hint: When shooting for hours in very low temperatures, the camera’s battery will quickly seem to die. Carry a spare and switch to it when that occurs. The “dead” battery will revive in about ten minutes if kept in a warm inside pocket.
The opposite problem can occur when you travel in a warm climate. When you move from a very cool, dry (air-conditioned) environment to the hot/humid outdoor air, condensation will form on the equipment. It can take a long time to clear, and you won’t be able to take photos in the meantime. Again, the same preventative measure applies: keep cold photo equipment inside your camera bag for 15 minutes to allow the contents to warm gradually; this will prevent condensation from forming on it.