I got the Sigma 15mm f2.8 fisheye. A very nice lens, really well made.
Uncorrected
With the lens profile applied in LR 6.5
The 2nd pic had the highlights reduced a little. But this is just an illustration of what LR does when the lens profile is applied. It turns the fisheye into a rectilinear lens. Interesting! You can see the angle of view is reduced (obviously - you are applying pincushion, so have to crop) and my wild guess is a loss of about 10 degrees. so maybe that makes it a 17mm or so lens... still a good result given there are no modern 17mm lenses for the K1
As regards quality, I have put some files on dropbox but if I post the link, DB will pull my account due to the traffic. So if anyone is interested, drop me an email. The pics won't be copyright and can be rehosted anywhere.
On the above pic, zoomed in 100%, this is the centre (I used centre spot focus)
f2.8:
f5.6
f8
The left corner, for the three apertures respectively:
So, lots of chromatic abberation and generally poor, but this lens was £360 and e.g. the Pentax 15-30 is over £1000...
The CA is easily removed in LR
As regards flare, here the low winter sun is shining into the front lens element
This pic shows two roadsigns; one in the middle and one blue one to the right
Here they are 1:1, f5.6 (f2.8 is very similar)
again after the purple CA removed in LR. The two roadsigns should be about the same distance so there is a significant loss of sharpness to one side.
However much of that loss of sharpness is due to LR's pixel replication in the lens correction. This is the blue sign, 1:1, from the non-lens-corrected image