Originally posted by RobA_Oz Interesting observation. Having a lens purported to be weather-sealed or something like it tends to make one perhaps over-confident in the absolute truth of the description. Thinking about the body-lens combination (which I have), if the seals on the lens are good, zooming in and out should displace air drawn from (/through) the body, which is also sealed. Nature abhoring a vacuum means that there must be a vent somewhere, putting aside the possibility of the air resulting displacement simply lowering and raising the air pressure in the assembly. Have to think further on that one.
A lens that changes its length can never be airtight. Change focal length when under cover or surrounded by your jacket or whatever, that's just a shooting skill.
A lens like the DA*50-135 doesn't change its length, so that's more rugged, but on the other hand, all such lenses suffer from more focus breathing, too. Only at infinity do you enjoy the whole focal length.