Nikon 15mm f3.5 AIS v Sigma 12-24mm f3.5-4.5
Here we go again: perhaps the most important conclusion to derive from this test is the critical importance of the correct adaptor – the wider the lens, the more important it is to get it right.
It would be easy to buy a Nikon 15mm, test it against the Sigma, and conclude that the game wasn't worth the candle. I did. And with a too-thin adaptor, you would be right. The Sigma 12-24mm's appeal is how well behaved it is: uniquely versatile, practically zero aberrations and, stopped down, moderately sharp. It's hard to better.
But, properly adapted (FotoDiox Pro recommended) the Nikon 15mm f3.5 AIS is significantly sharper at all apertures, and also extremely well geometry-corrected. It does require shading to avoid flare resulting from light obliquely striking the bulbous front element, and some post production to fix the CA, but it is capable of producing results beyond the Sigma's ability - for a fraction of the outlay of the Zeiss or Leica 15mm.
How it compares to the Zeiss and Leica will be the subject of a future test....