Dust factor in push pull vs twist zoom

This could be a dumb question, but whether you twist or push pull a lens to zoom, isn't the end result that the lens extends or retracts? If so, what is the difference between method used to do this as far as dust getting inside? I have the 28-135, a twist, and it extends. I also have an older 50-200 push pull that extends and I can't figure out why the big deal.

They all don't get longer or shorter, but they all move disks of glass inside the tube for zooming and focussing. And as far as I know, none of them are air-tight.

I suppose if one does a lot of shooting inside a vacuum cleaner bag this is a problem. The thing to do is avoid that.

Will

Michael Singewald said:

...whether you twist or push pull a
lens to zoom, isn't the end result that the lens extends or
retracts? If so, what is the difference between method used to do
this as far as dust getting inside?

Click to expand...

Michael

as soon as the rear element of the lens moves it will of course displace air inside the camera.

The 100-400 push/pull is displaces much more air of course because of the length and size and therfor people notice it when zooming. On a 28-70 the amount of air displaced is so small that you generally dont notice it but of course it does move air.

--Michael SalzlechnerStarZen Digital Imaging http://www.starzen.com/imagingE-10 / D30 Photo Albums http://albums.photopoint.com/j/AlbumList?u=1605723

In addition to what Will said,

To make a zoom that does not extend, the length of the lens barrel gets set by the maximum focal length and focus distance to a large degree. If say the a 100-400mmIS lens did not change length, and acted like the say 70-200F2.8L, it would end up with a lens barrel about the length of the 400mm position and be very unweildy to store and take with you.

Then you get into whether a rotating mechanism versus the push pull is better for extending the lens barrel (a different set of trade offs), but the air pumping would be about the same.

Karl

Will Perlis said:

I suppose if one does a lot of shooting inside a vacuum cleaner bag
this is a problem. The thing to do is avoid that.

Will

Michael Singewald said:

...whether you twist or push pull a
lens to zoom, isn't the end result that the lens extends or
retracts? If so, what is the difference between method used to do
this as far as dust getting inside?

Click to expand...

Click to expand...

--Karl

Here is my theory about all this, all volume of air moved when zooming, the 100-400 for example, could not possibly be passing thru the body of the D30, it leaks, by design, from the joint between the two lens sections. These joints usually are felt lined and filter dust. If the lens were airtight and the body not, you would hear the air rush out all the little sections of the D30 body. This is not the case, the body is much more air tight than the lens (to prevent dust infiltration to begin with) ... and while I'm sure a small amount of air goes in the lens mount, most all is expelled thru the lens itself. Otherwise, our camera bodies would be acting like air filters and would be chock full of dust all the time.