So, I was in Juneau Alaska (think temperate rain forest), and ran into a double condensation problem.

The first issue I ran into, I do not have a specific timeframe. My X-T1 developed condensation on the inner surface of the viewfinder (ie could not wipe it off). There was none on the exterior of the viewfinder. It did not mitigate for hours and only once I was indoors.

The second issue I ran into was condensation inside my 18-135. In this case, the lens had been on the camera and outside for more than an hour when it developed. There was no condensation on the exterior surface of the lens. The weather conditions were constant throughout. Strangely, the condensation was in the center of the lens and that was surround by a hexagonal pattern of condensation. Also, this did not resolve until indoors.

My concerns are twofold. One is the fact that there is moisture inside the lens that is condensing. When it dries, I would suspect some residue as well as the potential for fungus.

The second is that the viewfinder again has moisture inside it, and it became unusable when it was in this state.

Now, I've dealt with external condensation all the time. It doesn't make sense to me that there would only be internal and not external condensation, and even though the camera and lens are obviously not hermetically sealed, I wouldn't have expected this.

For the record, my 55-200 and 35, both in my bag in the same conditions, did not show any condensation internal or external.