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Originally Posted by Tannin
(Wasn't that an animal in the Dr Doolittle books?)
What about dust? The dust pump argument is a nonsense. A moment's thought will demonstrate that, for any given expansion ratio, a given volume of air must be moved: it doesn't matter if the lens screws out or slides out, it still has to move the same amount of air, and still has to suck in the same amount of dust. The dust question only becomes relevant when you are considering an internal zoom lens of fixed physical length - in which case it applies equally to push-pull or twist-zoom units.
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Hi Tannin
I have a different opinion, having owned two Canon 100-400 lenses over the past 18 months and they do suck in dust mainly down to poor seals. Your are correct in the movement of air theory but hold your hand at the base of the zoom when you zoom in and feel the rush of air. I nicknamed mine the dyson. In fact the seals on mine were that bad that on a recent balloon trip moisture got inside the lense, it was damp and very cold though.
Second problem is the mode 1 IS on the lens cannot keep up with a fast moving subject on the later Canon DSLR's, resulting in error 01 and camera lockup. After Canon technicians admitted this to me I decided to swap to an internal focus Sigma 100-300 EX and now my sensor is clean again. Supplemented by a nice Canon 400L f5.6 prime which again reduces dust ingestion. Canon are planning to replace this lens with a second generation IS as they have done on other L stabilised lenses.
I am glad you are happy with your choice as I am with mine.