How does dust get in a lens?

Question

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.

ALL lenses have dust in them, even before they leave the factory. If you don't believe me, maybe you'll believe Uncle Roger: The Apocalypse of Lens Dust. But beyond that, "Fully weather-sealed" does not mean what most people seem to think it means. It does NOT mean "hermetically sealed." — Michael C, Jan 15 '19 at 19:15
Related: Is it normal for there to be tiny dust particles in a new lens? and Is dust in a zoom lens a problem which should be addressed? and What makes a camera 'weather sealed'? and How do I avoid dust entering my camera when changing lenses? and Will dust inside a lens affect the image in any way? — Michael C, Jan 15 '19 at 19:57
It does NOT mean "hermetically sealed." Indeed. The new version is specified to be dust sealed too, not this one though. — Orbit, Jan 15 '19 at 20:23
Does make one wonder, why has no manufacturer tried going all motor-driven internal focus, hermetically sealing the lens and filling it with dry air or inert gas (not vacuum, implosion risk :) , as is these days done routinely eg in mid range hard drives? — rackandboneman, Jan 15 '19 at 20:41
@rackandboneman Because dust in lenses has pretty much ZERO effect on image quality. — Michael C, Jan 15 '19 at 22:04

GhislainCote · Accepted Answer · 2019-01-16 15:50:32Z

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 !

"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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