Unable to realize the potential of Nikon D800

I had the intention zuzulegen the Sigma. I made about 20 screen tests mostly with open aperture and minimum and maximum focal length.
After some consideration, I still went to the offer of a merchant, the Nikon to taste (I was not only because I knew how that would run out). Well, I lifted the same motives from the same exposures.

Result:
- In the focus the Sigma can not exploit the potential of the D800 (36 megapixels). When zooming into the picture, the Sigma is noticeably blurred, the "sharp crack" Nikon. The Focus takes the Sigma toward the outer edge, more so than at Nikon. In practice, this should usually be less relevant.
- The Vignette "vignetting" I could not find the Nikon, Sigma when it is available, although not disturbing for me because most subjects are located in the central area.
+ With a sun rays is metal railing against a dark background, the Nikon exhibited at extreme magnification of a purple-transition to the high contrast areas, the Sigma this chromatic aberration was significantly lower.
- The color fidelity of the Nikon is really much better. In practice, this is however not really stand out for lack of comparison.

The differences were already noted on the monitor screen, the computer at home, they were then seen clearly.
As the trial proceeded Nikon can certainly imagine - my wallet is only half as heavy as planned ...

Overall, the value for money of the Sigma is still much better and probably really let the differences only with a high resolution and quality cars like the D800 make - but those who invest so much in a body ...

On the D800, I would give the lens 3 stars.

I must add that the experiments not withstand laboratory conditions. They are merely arisen from practice. I did not use a tripod, not exactly marked positions and in some cases the automatic white balance. Only the aperture-ISO combination was identical.