What's a good protective lens filter?

joedlh wrote in post #16798869
Did you ever look closely at the front of your lens where the filter screws in? The lens retaining ring leaves no air gap or moving surfaces that would allow the elements to get inside the lens. So putting on a filter does absolutely nothing for weather-sealing. The elements are going to get in at the focus and zoom rings. That's what you should worry about.

joedlh wrote in post #16798877
Just curious. Why does it require a filter?

It's not the zoom or focus rings that are the issue here. Those parts are sealed otherwise Canon would not say that their L lenses are water or dust resistant. The issue is that for the 17-40 and 16-35 lenses (not sure if other L lenses as well), the front element moves in and out when you zoom, creating gaps where water and dust may enter the lens. You can see what I'm talking about in the gif at the bottom of this web page:

http://vahonen.com …ent/lenses/Cano​n17-40F4L/

For appropriate situations I use THIS!

hollis_f wrote in post #16798876
Er, so why do the manuals for some lenses (e.g, 16-35, 17-40) include the wording quoted by mag10?

I stand corrected. No lens that I ever owned had a moving front element inside the filter ring.