Don Haines said:
The 400F5.6 is a constant length lens, and as such, does not pump air in and out with zooming. If you are in a dusty environment or salty sea air, this will help to keep the insides of your camera clean....
Don, I'm not picking on you. It's just your quote provides a handy reference.
I would love to see some authoritative source or reference for the "pumping dust" onto the sensor stuff. All I can find doing a quick internet search are similar discussions on similar discussion boards from people who say this without any proof other than they have dust on their sensor and they think it is because of a zoom lens.
I trust LensRentals.com and the only thing I could find on their blog was a general statement regarding lens dust, where they said they don't notice any more dust on zoom lenses than on primes.
If people are going to make the claim that zooming can push dust into the camera body and then onto the sensor, I'd like to see some backup for that other than anecdotal evidence.