Clear optics, fogs up - Nikon Prostaff 3-9x40 Rifle Scope, 1in Tube

I purchased this scope because it was on sale and I needed a low price, good name scope to mount on my AR for deer hunting. I have a 6x18 Nikon Buckmaster on my bolt gun and I really like the clarity of the optics, so I bought this scope. This scope also has clear optics but I had problems with fogging on several occasions. However this did not stop me from shooting a deer in low light conditions in the evening. I think this is a good scope for dry conditions without much temperature change, but it fogs up when used in the cool and often damp conditions you encounter in deer hunting. Maybe I just got a defective one.