Get Rokinon 12mm f/2.8 Fisheye at B&H Photo. Available for many brands.
Rokinon has aggressively released lenses for Sony mirrorless that are really DSLR lenses by adding a longer barrel to the DSLR lens, essentially an extension tube taking up the gap that the mirror box on a DSLR would occupy. On my recent trip, I tested the Rokinon 12mm f/2.8 ED AS IF NCS UMC Fisheye for Sony E-mount (and also the Rokinon 35mm f/1.4).
I like a fisheye prime lens; it can do jobs no other lens can, and it is sadly neglected by Canon and Nikon* and Sony (Panasonic makes a very fine 8mm f/3.5 fisheye for M4/3). I wish Zeiss would produce a very high grade APO fisheye. Alas, my desire has not come to pass—the market doesn’t appreciate a fisheye very much, apparently, so no one makes a really good one.
I’ll be reporting on the Rokinon 12mm f/2.8 fisheye with my usual high-res examples soon. In the meantime, here are some images I had great fun making. A fisheye in wide open spaces is especially useful, and when used well the fisheye effect can be not so obvious.
* The Nikon 16mm f/2.8 fisheye is not inspiring. The Canon EF 15mm f/2.8 fisheye is decent, but needs work. The Canon 8-15mm f/4L is excellent, but relatively bulky and its f/4 aperture makes it largely useless for night shots.