As I understand it here are the physics: Air contains moisture, warm air contains more moisture than cool air, as the air cools it will reach the dewpoint. The dewpoint is the temperature at which the air is saturated with water and can not hold any more. If the temperature lowers any further the water will be condense onto the nearest surface. In your case it was your lens. It was also collecting on any surfaces, the legs of your tripod, rocks nearby etc. If you can keep your lens surface temperature above the dew point water will not condense on it. Near a lake you can be sure the humidity was high therefore a higher dew point. If you warm your lens once it is outside this will help though don't do it with anything that puts more moisture into the air as example a propane heater which is producing more moisture as it burns. Use an electric sorce if possible. There are products which are applied to a lens to reduce fogging. I do not reccomend putting them on a camera lens as the may be harmfull to the coatings and difficult to clean off. I have used them however on the eyepiece as an extreme measure and found that they can be helpful.