A word of CAUTION!  Cameras and lenses are VERY susceptible to damage from condensation getting inside.  This happens when the cameras or lenses are moved from warm to cold climates, or from cold to warm.  Condensation can cause rusting of the electrical components, and mold forming on the internal glass.  Both problems are very expensive to fix. 

Follow These Steps to Prevent Lens Condensation 

1. Before changing climates, place the camera and lens into a large zip-lock bag. 
2. Squeeze the extra air out  of the bag, and seal it tight. 
3. Move inside, or outside as needed, but DO NOT open the bag for at least 15 minutes, more if the temperature difference is great.  This will allow the camera, and lens to adjust to the new temperature without causing condensation. 
   This technique works because when you squeeze the Zip-Lock before sealing it, you remove the moisture-laden air that would cause condensation when the temperature changes. 
4. When you feel that the camera, and lens have adjusted to the new temperature, you may open the bag and remove them. 
5. Use the same process before you move back inside or outside, which ever is the case. 

TIP:  As a general rule, I leave at lease one camera locked in my car for use on cold mornings.  Since it is not heated, it doesn't need to be protected from condensation.  That way, I can get up before dawn, drive to a predetermined location, and start shooting as soon as I arrive.  No need for the zip-lock bag.  If the gear changes from one temperature to another slowly, there is no condensation buildup.  The problem only happens when you take it quickly from a warm environment to a cold one, or vice-versa.