Flare Performance
Nikon 15mm f3.5 AIS v Sigma 12-24mm f3.5-4.5
The Nikon 15mm AIS has a bad reputation for ghosting and flare. Ken Rockwell's test pratically writes off the lens as unusable, claiming an invasion of blue blobs whenever light is present. I have to say that this sample wasn't troubled overmuch by these symptoms. Problems can arise when light strikes that bulbous front element obliquely, but the same is true of most ultrawides.
So here's how they perform with a strong, naked point light source firing straight at them from a distance of about three feet: first the Nikon:
Then the Sigma:
Note the relatively warm/yellow cast of the Sigma, and slightly better controlled flare performance. Neither could be considered problematic. Obviously shooting into the sun would induce stronger symptoms, but very rarely did I experience Ken's blue blob syndrome unless I tried to reproduce it. Even then, a judiciously positioned hand just made the problem vanish.