(Not quite the) Conclusion
Nikon
15mm f3.5 AIS v Sigma 12-24mm f3.5-4.5
A
triumphant hurrah for progress then? With some reservations, yes. Bearing
in mind the practicality the Sigma zoom affords, and its impeccable behaviour
with regard to geometry, aberration and corner-to-corner consistency, we'd
perhaps be churlish to complain about its relatively low resolution. Its
colour rendering is on the warm side (stopping just short of full blown
'Sigma Jaundice') but all told it's another impressive victory for modern
technology.
The
Nikon 15mm f3.5 AIS is not without merit, though. If only it maintained
the resolution of which it is capable evenly across the frame. The
problem is particularly acute at smaller apertures, as was the case with the Nikon 14mm - both lenses displaying a weird rippled, toroidal plane of focus (like a slice of doughnut)...
And you know what? At this point (somewhat late in the day) it occurred to me that the adaptor which worked so well on the Nikon 28mm might not work as well with the ultrawides. A similar problem can occur with adapted Contax lenses, as Pham Minh Son discovered...
So the tests were re-run at f5.6 and f11 with the thicker FotoDiox pro adaptor which has a deeper register than the Japanese-made items I customarily use with longer Nikon glass. Observe the difference:
More results (and the final conclusion) on the next pages...