I'm not going to dump on the Nikkor 15mm since I have not shot with it personally, but I would be curious to see the MTF graph for it after reading a description of it on the 16-9 website. This description is quite telling:
Talking about the 15mm Nikkor "The problem is particularly acute at smaller apertures, as was the case with the Nikon 14mm - both lenses displaying a weird rippled, toroidal plane of focus (like a slice of doughnut)...". Quite amusing...
Looking at some sample images, the corner sharpness is abysmal at any aperture:http://www.16-9.net/lens_tests/15mm_2/sigma1224vnikon15b.html
The LS55 sample file from Doug (full frame sensor and more demanding on the lens) while minimally sharpened, appears vastly superior to the results from the Nikkor lens. I doubt either one would cause any focusing problems in a viewfinder but for 40" prints I doubt the results from the Nikkor lens would be very satisfactory.
This is obviously a pointless comparison, but then again so is this thread.
I'm not up to date on copyright information, but using Guy's long exposure image that contains motion blur from moving branches and selectively focused on the foreground with an out of focus background, and comparing it to small images that easily mask a lens' flaws is a bit of a cheap shot. But then again, so is fabricating MTF graphs and drawing red circles to make everyone believe that an 80MP razor sharp image at 40" would appear out of focus in a 12" magazine spread.
I don't question Fred's skills as a photographer, and it isn't my goal to discredit him, but some of the claims made to discredit a perfectly good product are simply outlandish (yet extremely amusing).
Steve Hendrix originally started the thread to share some useful information with everyone. Some jumped on the opportunity to blast a good product, proof was then shown that the lens was pretty damn sharp (even with any mild file sharpening applied). And once again it's all headed downhill fast; I suggest the original poster lock/close this thread before it degenerates any further.
And if anyone wants a copy for the archives, I'm sure Fred would be thrilled to send you a PDF
Lets go back to lens image quality.
Lets compare some images to the Phase One 55mm LS 2.8 and 40MP crop sensor posted in this thread.
D800E with Nikon 15mm