Phenix205 said:
Nice work Mark. Alex Nail has some comparison of center and corner sharpness between this lens and 16-35II, 17-40, and 17TSE. The sharpness of the 16-35 IS is on par with 17TSE. Good to know.
http://www.alexnail.com/blog/reviews/review-canon-16-35-f4l-is-vs-16-35-f2-8l-ii/
The extra sharpness from f/4-f/8 is great on the f/4 IS. At f/11, it looks like a draw between everything except the 17-40 - I see no significant improvement in the 16-35 f/4 IS over the 16-35 f/2.8L II at f/11. The writer only very briefly mentions one of the significant advantages f/2.8 can give you in a number of scenarios, such as increased light, lower ISOs, higher shutter speeds, and thinner DOF. I also disagree that the flare is any better on the 16-35 f/4 IS than the 16-35 II, it looks about a draw.
And, I do think the 16-35 II's sunstar is quite a bit superior, despite this blogger's opinion; the best way I can describe it, the 16-35 f/4 IS sunstar looks like a picture of the sun, while the 16-35 II sunstar looks like the sun as I see it in real life. If I am taking a landscape picture, I want majestic and realistic looking sun rays. I would go as far as to say I would find it difficult to upgrade to the 16-35 f/2.8L III if it has sun rays similar to the 16-35 f/4L IS.
For strict landscape shooters if you have not purchased a UWA yet, the f/4 IS looks like it will probably be your best bet to allow you great sharpness from f/4+, and similarly a much needed upgrade if you have a 17-40L. If you already have a f/2.8L II, it is a muddy proposition. You give up f/2.8 and the 16-35 II's unique sunstars for a sharpness increase that is only readily apparent from f/4-f/8, with differences becoming minor at f/8. And, if you are an event shooter or do both events and landscape, I'd definitely recommend the f/2.8L II over the f/4 IS for the much needed f/2.8 and similar quality at f/11.
Thinking about that for a second, if you are an event shooter, the majority of your shots will be at f/2.8 for increased light. On the other hand, if you are a landscape shooter, f/11 is extremely popular as it is the narrowest aperture/greatest DOF that does not suffer the greater sharpness losses of smaller apertures due to diffraction. So, arguably, the place where the f/4 IS shows the biggest sharpness difference is the middle of the aperture range (i.e. f/4-f/5.6) that will be used but are probably not the bread and butter for either event or landscape shooters. While it is sharper in those middle apertures, is it worth losing f/2.8 and those awesome 16-35 II sunstars for that? Some may think so, but others may not. But, it is worth pondering before you go through the cost and hassle of swapping lenses (and potentially filter sets).