This is almost instant , Like 10-20 seconds, from heat transfer from my eyes to the lens. Both binoculars and scope. This is the first year I have noticed this problem.
First time I noticed was pre season and I can't hold a bead while breathing so I know I held breath my breath as best I could on a trotting coyote one morning until it disappeared into the foggy lens. I cleaned them with alcohol wipes and they seemed to get worse after that. Thinking there must be some sort of lens cleaner/protectant to help with this ?