Quote Originally Posted by Joel Bookhammer
So I have been debateing between the 400 5.6 and the 100-400.

I'd go with the 100-400 for IS and versatility. IQ is a little better with the 400/5.6, though.


Quote Originally Posted by Joel Bookhammer
With the Push-Pull Zoom mechanism the air has to come in and outfrom inside the lens, with that there would have to be dust. (...) Is there some type of "seal" that helps keep the dust out? If there isnt a seal, it would seem to me that the push pull zoom would be a dustpump of sorts.

The push-pull is a "dust pump of sorts"; any lens that changes length is. The only lenses that are "sealed" against dust are the weathersealed lenses. Of course, a lens that zooms internally (like the 16-35L and 70-200L), also keeps dust out well. For some reason, the EF-S 17-55/2.8 IS seems to be the only lens that is known for collecting dust.