Twice since I've come back to NZ from overseas I've had 2 of my Lens steam up inside, my 24-70 L and 16-35 L. The 24-70 would make sense as I did use it in the rain in Hong Kong but since its both of them... hmmm... The other photographer shooting a show with me last night checked it and suggested it could be that the lens was freezing cold and the venue was really hot. So do L lens steam up if the environment change is severe enough? These lens have performed flawlessly until I started storing them in my Lowerpro backpack. Maybe they're coming out too cold?