'"Of course, the high filter cost can be avoided by simply not buying a filter. But, the 16-35 L II is fully weather sealed to protect against dust and moisture only when a filter (typically a UV Filter) is in place like all of the other current-at-this-time sealed non-super telephoto lenses). Even though the extending inside portion of the lens barrel is gasketed, This filter is necessary to fully seal the lens. The lens does not change in overall size, but the inner barrel extends to its maximum near 28mm from its minimum at 16mm. A filter completely encloses this movement - and does not rotate........ "" source: http://www.the-digital-picture.com/R...ns-Review.aspx

like some seniors here have mentioned, it depends very much on whether you want to protect your lens. in this case above, the lens here is rather ex and i would diffinitely protect it with an expensive filter.