It's a pity that threads like this, that are in praise of a particular lens, do not show images comparing the lens with another lens, such as a popular zoom lens used at the same focal length, so the potential buyer can get an idea of the practical significance of any improvements that the prime lens may have in relation to lenses already owned.
As we all should know by now, it's the photographer who counts, more than the equipment, and any good photographer, such as Bernard or Quentin, can take interesting and impressive photos using virtually any lens.
The reason I raise this issue is because my copy of the Sigma F1.4 does not
appear to be sharper in the centre, than my Nikkor 24-120/F4 zoom at 35mm (or more precisely 33-34mm which is a more exact match).
Initially, I considered returning the lens, but it does have some good points, and I think it's now too late to return it.
Comparing the lenses on my D7100, I would say the good points are:
(1) The Sigma seems as sharp in the centre at F1.4 as the Nikkor zoom is at F4.
(2) At F8, the Sigma is clearly sharper in all corners than the Nikkor at F8, but not
noticeably sharper in the centre, and only marginally
sharper at the borders, as opposed to the corners.
The puzzling aspects, which make me wonder if I've got a lens (or camera) at the lower end of the quality control spectrum, is corner performance at F4. In the lower left corner, the Sigma is clearly very much better. However, in the upper right corner, the Nikkor zoom is clearly much better.
Resolution at the middle of the lower edge, and middle of the upper edge, camera held horizontal, is about the same in both shots, implying that misfocussing is not the cause.
Being aware of such characteristics and flaws is going to be useful. If I'decide to use the Sigma for stitching a landscape, in place of the Nikkor zoom, focussing at or near infinity, I'll know that little purpose will be served by using the lens at apertures wider than F8.
The main use of this lens will be merely to get a shallower DoF than my Nikkor zoom can provide, and doing so without loss of resolution in the plane of focus, compared with the Nikkor zoom.
If the Sigma boasted VR, I could also claim its wider aperture would be useful for hand-held photography in low light. Unfortunately, the difference of three stops between F1.4 and F4 is hardly better than the advantage of the VR of the Nikkor 24-120 zoom. It will be useful to freeze subject movement, though.
If anyone feels convinced that my copy of this Sigma lens is below standard, please post some comparison shots, preferably taken with a D7100. It might be the case that my camera rather than the lens is flawed. I did have to make a substantial 'AF Fine Tuning' adjustment of +15 to get the Sigma lens to focus accurately.
PS. I should clarify things before anyone picks up my mistake. The attached images have been mislabeled. The camera is the D800E, not the D7100. It would be very surprising if such big differences existed in the corners of the cropped format.