The 100-400 was a push-pull lens and there was a myth surrounding the lens that claimed that this design drew more dust in. It didn't, but myths don't need facts.That's true in my limited experience - I've never had an issue with multiple externally extending lenses. But, I guess it depends on how much dust we're talking about. I recall people referring to the EF 100-400mm f/4.5-5.6 as an 'air pump', and being actually disconcerting to use. Yet it was also meant to be an exceptionally good lens through-out the range.