fish_shooter said:
I try to only swap out parts while indoors while in dry heat. This way the camera interior is relatively dry so even when cooled will not lead to condensation.
Bodies and lenses – even weather sealed ones – are
not hermetically sealed. Weather sealing keeps out dust and moisture, not air. Moisture is liquid water, 'humidity' is (a measure of) gaseous water dissolved in air. That means individual water molecules floating around in the air, and those water molecules are smaller than the nitrogen molecules that make up most of the air. So if there's humidity in the air, it'll be in your gear. If the temp warms up too fast across the dew point, it will condense inside.
In winter, the issue is usually going from cold, dry outside to warm, less dry inside. In that case, it's not the water vapor inside the gear, but the external water vapor that condenses on the surface of the cold gear. In that case, taking a plastic 'ziploc' bag out so it's cold, then putting the gear in it before bringing it in, so the condensation forms on the surface of the bag. Allow time for the gear to warm up - using it, especially an extending zoom (even a 'weather sealed' one) will draw the warm, more humid air into the cold camera/lens interior, where the water vapor will then condense.