Hi all, I searched this topic but couldn't find anything on it. I purchased a 100-400L ii nine months ago and there is now quite a bit of dust under the front glass. I know this will not affect image quality, and I do pack it out in the hills while hiking so it does see a lot of use. I do have a UV filter on the front. Is this a common and to-be-expected problem?
Lenses can have dust brand new. Since focusing and zooming have moving parts dust can work its way in its just a fact of life. If you have a extending barrel or front element it will be more susceptible. And profession grade lenses such as the L's which have weather sealing will be less susceptible, but if used in dusty environments I would expect that the dust will work into the rubber seals over time.
I believe this is a push-pull zoom lens, there is a breathing hole made into the lens barrel to allow it to "breathe" and therefore using this lens in dusty conditions is not a good idea. However, it is what it is and so therefore its a thing you must live with. An occasional trip to a repair station to clean it out would be helpful. I used to have one myself and I couldn't stand it so I let it go, I now shoot mostly primes. Good luck and take good care of it and it will serve you well.
Thank you! My anal-retentive side will have to work with it. I was wondering if it could be cleaned if sent to a repair shop, and apparently it can. Thanks for the info.