Originally Posted by Larry
Ahhh the Internet. Fountain of Truth. []
The 24-70mm f/2.8L and 24-105mm f/4L IS are weather- and dust-resistant lenses, so they have internal seals around the zoom and focus rings which non-sealed lenses lack. However, the 17-55mm is no more prone to dust than any other zoom lens (and probably less than some, since the extension is shorter than something like a 70-300mm). Personally, I've never had a dust issue with mine. I will say that I always keep a UV filter (B+W MRC) on that and all my lenses - it's required to complete the weather sealing of 'sealed' L lenses, and I find it easier to clean a filter than the front element. On the 17-55mm, there are two small holes right near the filter threads, and I suppose that may be a source of dust entering and lodging behind the front element - so, a filter on the front may help in that case.
Even so, a little bit of dust (or a lot, for that matter) is not going to make a speck (pun intended) of difference for the images - light passing through the lens is defocused, meaning you won't ever see a spot from even large specks of dust. Lensrentals.com has a nice writeup on the effect of a front element 'scratch,' and internal dust will have much less impact (sorry, another pun) than that.
Bottom line, don't worry about it, it's a wonderful lens.
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