I'm bringing some of my post on from the thread that went a-wall. Dust and a zoom has to do with Large volume changes in air.
Separate this discussion into two completely different zooms:
1) Telescoping Zooms like the 18-200mm
and the
2) Internal Zooms like the 70-200mm.
The Telescoping zooms many times act like a plunger, it moves in and out and the volume inside the lens barrel changes significantly. It takes Speed and a large Volume change for this to happen. Some dust does get in due to this. 3rd party lenses are much more prone in my experience as Nikon does a better job sealing their lenses.
Internal Zooms lack the volume change of air that creates the suction and the subsequent push of air and dust to a sensor. To move a significant amount of dust, the whole lens casing has to move like the 18-200mm not just a couple of elements like the 70-200mm. Even with the rear element moving, there is just not enough of a volume change for any real suction to happen.
Primes are no different than Internal Zooms - they do not suck any dirt in.
Think of it like a toilet plunger. It has a cup on the end that when you push it in, it collapses and what was in the cup is pushed out. The larger the volume of the cup, the more force you have pushing out. That is like a Telescoping zoom.
Take that same plunger, cut about 2" of the ring off and drill about 20 large holes in the remainder of the cup. When you push it and the cup collapses, what volume is there passes through the holes with very little if any exerting downward into the drain. This is just like any Prime or Internal Zooms. Even if the rear element moves, it does not move enough to create a vacuum that is needed to suck or blow dust in.
Take a your lens, prime/zoom and as you focus and zoom it, put your hand at the back and see if you can feel any "wind" or "breathing" from the lens.
