Fisheyes for Night Photography

I've been looking to add a fisheye to my arsenal of night photography lenses, but I was never able to locate solid information on what was the best lens for my style of night photography specifically. I knew I needed to do my own testing to know for sure. I received the Sigma 15mm 2.8 and Nikon 16mm 2.8from BorrowLenses and decided I needed to go someplace spectacular to test them out, what better place then Canyonlands National Park?

I first tested the Sigma, I tried setting the focus manually to infinity and taking a shot only to be disappointed to see blurry stars on the LCD. I then used live view to set my focus on a bright star, even though the focus was set well before infinity it was now razor sharp. Not ideal, but I have the same problem on my Rokinon lenses, so I can get used to this. I switched over to the Nikon which feels solidly constructed compared to the Sigma, is smaller and lighter as well. Another positive of the Nikon is the hard stop at infinity, this means no guessing on your focus, just spin it until it stops and you have no worries. Sounds like the Nikon is the clear winner right? I thought so until I loaded the images at home to get a closer look.

All images taken at ISO 3200 f/2.8 30" - Sigma on the Left, Nikon on the Right
100% Crop in the center of the image; the landscape looks nearly identical, both very sharp. The stars are a different story though, the Nikon is considerably softer.

100% Crop at the top center, the Nikon is getting worse and the Sigma still looks incredibly sharp.

100% at the top right, the Nikon just fell on it's face, even the trees are blurry here, completely unacceptable for me. I didn't test it, but you would probably have to stop down to f/4 to get the sharpness back in the Nikon. As you can see there is very little coma present in the Sigma, the optical quality is top notch.

Ultimately the Sigma wins for me, image quality is my top priority. That said, you may wish to choose the Nikon because a lower quality in-focus shot is way better than a high quality out of focus shot. The Nikon is just so easy to use with the hard stop at infinity. It will be much harder to miss a shot and you'll enjoy using it more.

Oh, did I mention the Nikon is $300 more? The choice just became even easier. 
Sigma 15mm f/2.8 EX DG Diagonal Fisheye Lens for Nikon AF - $609.00
Nikon Fisheye AF Nikkor 16mm f/2.8D Autofocus Lens - $899.95