Here are a couple examples, but don't read a lot into them yet -- they were shot handheld and processed on my MB Air, AND I forgot my CF reader for the raws, so these are the in-cam jpegs :facesmack: Fortunately I leave sharpening and NR set low. Oh, it's also fairly breezy today and the vegetation is moving...
Oh, a bit of background: The lens is sharp. The veiling effects of diffraction are readily visible across the frame at f8 on the D810.
First let's look at an f5.6 shot -- focus is on the buoy center frame:
Crops, center, upper right corner and then left edge:
Now a more typical type shot wide open to show its character:
Comments: Firstly, the lens is sharp, even centrally wide open which is in itself pretty remarkable. Obviously the outer 1/3 to 1/2 suffers, as does near infinity subject matter across the frame at f1.4, but then that's not a typical use. (If you are an astro photographer, more serious testing would be required for that application.) At f5.6 it carries the sharpness very well, but not perfectly into the extreme corners -- but it is one of the best 24's I've seen in this regard. Centrally at 5.6 it is sharper than both my 17-35 zoom and 24 PC-e, but frankly, I don't feel it's by a significant enough margin in the center 2/3 usability to justify its purchase over the others. The PC-E actually carries the extreme corner a little better due to its significantly larger IC, though not that far ahead of the Sigma; and of course, the 17-35 is outdone by either prime 24 at the outer 1/3 edges and corners.
The one thing that surprised me was the amount of aperture isolation rendered at the relatively moderate subject distance of the pine cluster shot at f1.4. That cluster was about 30 inches (75 cm) distant, and both fore and back falloff is visible with pleasant bokeh.
Is it a keeper? Yes, if you need/want a stellar performing fast 24, and can live with the bulk of a fast lens. For me, it's first a landscape lens, and I will use it for such. For travel, I like the effects I can get with f1.4, but overall size is not far off my 17-35, and the wide zoom is more flexible, so still a bit of a conundrum for me here and will need more time with it to determine.