Image Quality

In the following sections we will use DxO data and real life samples to determine how the Sigma and the Nikkor lenses compare to one another and to determine just how the Nikkor 24mm performs.

Sharpness

   
Full-Frame The Nikkor 24mm performs very well in terms of sharpness, although it does appear to drop off at 2/3 the distance to the corners when shot wide open at F1.8. The corner sharpness improves a great deal as you stop the lens down, reaching its max sharpness at around F8. Center sharpness is excellent and fairly consistent across all apertures before F8 from which point diffraction limits its performance. Center sharpness is best when the lens is stopped down to F5.6.
Chromatic Aberration Wide open, at F1.8 the Nikkor does suffer from some CA in the corners with minimal amounts of CA present in the center of the image. As you stop the lens down the CA decreases a bit in the corners. The CA seen here is relatively easy to correct with your favorite post processing software.
Vignetting Vignetting can be a bit of a problem when the Nikkor 24mm F1.8 is shot at wide-open with an almost 2 stop decrease in light at the most extreme corners of the lens with the majority of the corner experiencing about a 1 stop decrease in light compared to the center of the lens. The vignetting improves significantly by F2.8 and then all but disappears when you stop the lens down to an aperture of F4 and continues to perform very well thereafter.
Distortion As expected with a wide-angle prime, a bit of barrel distortion is present in the Nikkor.
Transmission or T-stop

The lens' F-number is a theoretical value, and the actual light transmission value, known as the T-stop, is always fractionally lower due to light losses within the lens. Lenses with more elements, like a complex zoom, tend to be slightly more effected. The Nikkor was rated as having a T-Stop of 1.8 and Sigma was rated as having a T-Stop of 1.7. 

Our findings contradict those seen in the DxO data; as the Sigma lens does offer a 2/3 stop difference in light gathering ability over the Nikkor.

How does the Nikkor 24mm compare to the Sigma 24mm HSM ART?

Our comparison lens, the Sigma 24mm F1.4 performs very well and, at comparable apertures, surpasses the Nikkor with respect to center sharpness, distortion and vignetting performance. In terms of sharpness, the Sigma's center sharpness just surpasses the Nikkor when the lens is stopped down to F1.8. The Sigma is a little softer wide-open, but slightly out-performs the Nikkor when compared at the same F-stop centrally. Corner sharpness in the Sigma does suffer a bit, but recovers nicely, in much the same fashion as the Nikkor 24mm F1.8 when the lens is stopped down to around F8. Overall they offer very similar performance.

Vignetting in the Sigma is nearly identical to the Nikkor wide-open, but it does improve faster, with nearly all vignetting eliminated by F2.8 so the Sigma is much better when both are compared at F1.8. When examining distortion, the Sigma fairs quite a bit better with a fair amount less distortion than the Nikkor 24mm. However both lenses perform well considering the wide focal length.

In terms of transmission, despite DxO's data, our findings show that the Sigma 24mm does indeed offer the nearly 2/3 stop difference you'd expect, compared to the Nikkor, wide open.

The Sigma experiences much the same pattern in performance with respect to lateral CA: it does initially suffer from some fairly strong CA in the corners which begins to improve once the lens is stopped down to around F5.6. The CA measured by DxO is lateral CA and in our real-world examples the CA seemed to be fairly correctable.

In order to get a better idea of how these lenses compare to one another in the real world we took them out to our balcony to perform a real world infinity test.