Canon has released the full technical report of the Canon 16-35mm f4 as well as the 10-18mm lens.
See the Tech report here: Canon Museum
My main interest here is the Canon 16-35mm f4 lens. I feel any real landscape photographer should be going for full frame or medium format, and that is the focus of my blog.
I have absolutely loved my 16-35mm f2.8L II lens that I've owned for a few years now. I've gotten several outstanding images with it and you can see my full review here.
Drawbacks Of The Old 16-35mm f2.8L II
My old lens definitely had weaknesses. Mainly, edges were not sharp, and quality fell off on the tele side of things. Basically the image at 35mm not good. So I would try to limit myself from ever zooming in. Although 90% of the time you'll find if you own an ultrawide zoom for landscape photography, that you'll be shooting at the wide end of the scale mostly.
What The New 16-35mm f4 Lens Give Us
Now with this new lens, they promise sharpness through out all focal ranges, as well as along the edges. A significant improvement and the MTF charts do not lie. This lens should be outstanding.
They also say there is an improvement to the overall durability to the lens. This might not mean anything but I can tell you about a few experience I've come across with the older model 16-35mm f2.8 lens.
First off, early versions of the lens had an all plastic rear mount that would often break. I've noticed Canon has upgraded the newer lenses to no longer feature this.
Also, current 16-35mm f2.8L II lenses have a real problem with sucking in dust. You HAVE to us a UV filter on the front or the lens creates a vacuumed as you zoom in and out. If you ever shoot anywhere dusty, like the infamous Antelope Canyon, or even a Color Run, your lens is instantly a mess.
Preorder: Canon EF 16-35mm f/4L IS USM Lens