After experimenting with a cropped fisheye lens, which I returned as the loss of view when attaching it to my full frame 5DIII was too much, I was pleased when Rokinon came out with their full frame 12mm fish eye: http://goo.gl/3c67oG
My first impressions are that this lens is a keeper. It is sharp and fun to use. Here are two sample images taken this weekend at the Acoma Pueblo: having bought a photo permit :-)
The first image shows an ‘un-fished’ view from the Acoma mesa, whilst the second image shows a defished view of the San Estevan Del Rey Mission Church at Acoma.
To get the most out of this lens, or any fisheye lens, you need to know how to defish it. After much reading I decided on a two part strategy. First I bought into FisheyeHemi, http://goo.gl/IlE7U, and second I use Photoshop CC's wide angle filter and/or the perspective transform tool.
Bottom line: a really fun and useful addition to my lens inventory and one I can recommend, especially for those with full frame cameras.