[popo]

I'm not sure it is simply external moist air that is causing it. The lens would generally be same temperature or warmer than the air so condensation from simply having it in the wet environment shouldn't affect it. Other lenses used similarly but kept externally dry-ish don't have the same problem. I suspect the 100-400L, due to its extension allows surface water on the zoom section to enter the lens, where it then evaporates and saturates more so than ambient. That is also a concern I have of the 70-300L but as said I now have one owner reference that suggests this isn't the case. Coincidently that user also has the same issue I have with the 7D joystick in wet weather, so I think our usage is quite similar. I find the 7D joystick can become unreliable after prolonged exposure to water.