I can just add to what Macguyver is saying. I have the Zeiss 15mm and 21mm and they are both phenomenal lens, especially the 15mm, which I totally fell in love with it this summer. But the 16-35 f4L IS is a lot more versatile and the IQ is very impressive. Be also aware that the Zeiss 15mm has a 95mm filter size, so filters are not exactly cheap.+1. I have the 15mm and have used the 21mm. The 21mm I hired a number of times, to get a feel of it (IQ is probably slightly better than the 15mm). But, I wanted something wider at f2.8, so I skipped the 18mm and bought the 15mm. Not a lens I use much, but, when you need something in that realm, it's magic.