Resolution (f5.6)

100% Crops (f5.6) Centre image circle

Nikon 15mm / f5.6 Sigma 12-24mm / 15mm / f5.6

Despite the immediately obvious difference in colour – neither are neutral – centre frame the Sigma zoom is clearly resolving better than the Nikon prime. However, recent testing of Nikon’s 14mm made me wary of trusting a centre frame crop alone. Wide open, the 14mm has a weird toroidal focal pattern that surrounds a soft centre frame with a ring of higher resolution before gradating back to poorly resolved corners. So it proved to be with the 15mm.

 

100% Crops (f5.6) c17mm from image circle centre

Nikon 15mm / f5.6 Sigma 12-24mm / 15mm / f5.6

17mm from the image circle centre, the Nikon looks much sharper, entirely reversing the centre frame results. The Sigma behaves as we might expect: resolution and light falling off in proportion to its distance from the centre of the image circle. The Nikon is playing by different rules altogether. As we move out to the extreme corners, however, we see the intial results reprised:

 

100% crops (f5.6) extreme upper right corner

Nikon 15mm / f5.6 Sigma 12-24mm / 15mm / f5.6

Allowing for the fact that the Sigma 12-24mm is inherently not a very high resolution optic, a healthy one turns in commendably consistent corners even at wide apertures. Or is it just that it looks good next to the Nikon 15mm?