So I have been debateing between the 400 5.6 and the 100-400. I have read a fair amount of reviews on here as well as asked for advice, but today I read something that I havent heard of up until now and never really thought of it.


With the Push-Pull Zoom mechanism the air has to come in and outfrom inside the lens, with that there would have to be dust. Has anyone experienced this? I had and still have a pentax lens that was used on a k1000 that was push pull, but havent used any others. Is there some type of "seal" that helps keep the dust out?


If there isnt a seal, it would seem to me that the push pull zoom would be a dustpump of sorts.


thanks


joel