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I recently saw someone offering an Nikon 70-200mm VR II lens for a good price second hand. The only drawback mentioned was that it has a little dust behind the front element.

How could this dust have gotten there, since this lens is a fully weather-sealed internal focus lens.

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Some possible reasons:

  • Intense environment (ex: The beach... especially bad if you change the lens on site)
  • Poor storage (dusty bag)
  • My Favorite : Zoom/Focus pump effect ! Even if it is sealed, a lens is not "a partial vacuum" ! The volume is changing, thus there is air going in and out !

For example : A lens like this old push-pull Canon Zoom can be "weather-resistant", but with such a MASSIVE push-pull displacement, there is a lot of air entering IN the lens.

The newer version of this lens can stay cleaner, because many more moving parts are staying in the body. At the other extreme, a typical 70-200 2.8 Zoom like this one will not move externally at all, and thus stay cleaner.

Most lenses will at least vary on volume with focus (even primes), thus having a problem with what is in the air around them.

The lesson here: Don't play with your zoom in the sauna !

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  • "Don't play with your zoom in the sauna !" Too bad, the view can be pretty nice. The zoom is a very good argument though, I forgot about about that one. Internal focus does not mean 'internal zoom'. Does that even have a name?
    – Orbit
    Jan 15 '19 at 20:26
  • You are right, what I meant is that focus is ALSO a factor, when the focus mechanism pumps in and out of the cylinder... So not even primes are totally safe from this.
    – GhislainCote
    Jan 16 '19 at 4:25

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