Bumping my own thread again. I pushed this lens hard up in the Uintas last weekend.
October Meadow by
ashergrey, on Flickr
Shot handheld, 1/25 of a second at f/8 and iso 800. IS was active, providing some extra protection against the low shutter speed.
The corner sharpness at f/8 isn't fantastic here, but it's much better than the f/2.8. Notice the lack of fringing on the bare tree branches in the upper right. I'm finding the Lightroom profile for this lens tends to err on the side of too little CA correction, but it's dialed in easily enough.
Again, not the greatest image but an example of the sunstars capable on the f/4. This was only stopped down to f/16, so you could probably get blades a bit longer by taking the sharpness hit going down to f/22. In general though, the specular highlights on this lens seem to be fatter in my eyes than those of the Canon 16-35 f/2.8L.
My general first impressions are this 16/35 f/4L is a nicer hiking lens than its faster, more expensive brother. I did notice the decreased weight when carrying it in my hand mounted to a Canon 6D. I don't have to reverse the lens hood to get it to fit in my typical camera bag (a Lowepro Toploader AW). I didn't expect myself to really want or use the IS, but I found myself shooting handheld well after sundown when I would have been relegated to the tripod or a much higher iso had I been carrying the f/2.8.