The Sigma 12-24 is just crazy wide on FF at 12mm. 12mm on FF is totally unforgiving for poor composition. In my opinion a tripod is really best, of course that is not always possible. I can not justify the cost of the DFA15-30 so I went for the Sigma.
You are going to have so e work in post cleaning up the distortion.
Sigma 12-24@12mm on the K-1.
Sometimes the border stretching can be used to an advantage.
I recently aquired a Tokina 20-35 Verizon 1. This is the version with a 72mm filter thread. After some overall testing I am happy with it.
Tokina 20-35@20mm on the K-1.
The new Iris 15mm seems to be a nice FF option but I have seen some really bad copies from both versions.