Thank you for your answer D.Sim.
I have a full format camera (5D mkII) and I got the impression of the cropping after reading the lens review on the-digital-picture.com
http://www.the-digital-picture.com/Reviews/Canon-EF-16-35mm-f-2.8-L-II-USM-Lens-Review.aspx
"Unfortunately, a normal thickness UV filter causes some additional vignetting at 16mm when mounted on a full frame body. This can be seen using the mouseover feature in the lens vignetting comparison. So, the B+W 82mm MRC Slim UV Filter is going to be the best full frame option in my opinion. Users of 1.3x and 1.6x crop bodies will be fine with the normal thickness B+W 82mm MRC UV Filter."
So I assumed that I needed the slim type and bought the one he proposed.
One thing that I did not forsee was that when you have a slim filter on the lens, you cannot put the Canon hood/lenscap? in front, as it cannot get hold. Instead B&W provided with the filter a rubber/plastic thing with Schneider written on it. This is not a problem per. se, but it seems like it could fall off during hiking with the lens.
But if I were to get a slim type for this 16-35 lens. Do there exist any ND faders that I can buy, and if so, where?