DUST MAGNET. DUST PUMP

[DUNKS]

Hi. Looking at a posting on here today I see the terms dust pump, dust magnet applied to a trombone/push pull zoom lens, and the lens condemned because of this. I use two of this type of older lens and do not get dust problems in my D300. I know that the action is a pumping one and we associate this with a pump, as in bicycle, but does this action on a lens actually pump air?
Imagine the lens as a tube, "it is" and place within it a plug which is not a tight fit, "as in a lens" and move the plug from end to end in the tube. Will it pump air? Also imagine a bicycle pump with the washer destroyed. Will this pump air?
I suggest one would have to put considerably more effort into the pumping action that ever we do to zoom a lens.
I personally think that this action actually pumping dust onto a lens is a myth.
I welcome other folks thoughts on this.
Because I had nothing better to do I did a little test today with my old 35-70 2.8 zoom with a push pull action. I attached to the camera end a fine membrane which was sealed."I just happened to have one in my drawer" and pumped the zoom ring from end to end as vigorously as I could. The membrane did not move at all!
Rude comments are not welcome!!!

 

[Canon Bob]

. I attached to the camera end a fine membrane which was sealed."I just happened to have one in my drawer" and pumped the zoom ring from end to end as vigorously as I could. The membrane did not move at all!

That's precisely the point, the "membrane" didn't move. This means that air is drawn in and exhausted through the seals of the lens. If the lens was sealed, then the "featherlite" dust particles wouldn't be an issue.

Bob

PS...your test equipment is very apt as I heard this type of lens being referred to as a "foreskin zoom" when I was down in Australia

 

[HoppyUK]

Put the whole camera and lens in a sealed plastic bag. Zoom the lens. See what happens to the bag.