I would like to answer your questions.
As far as I remember, it was f/8 on this particular photo (here without cornerfix):
if it is of any interest, I can upload the RAW file.
The bottom corner on the left look poor for me, the upper right corner looks better, but is more away. I use a very cheap M-adapter (only 38€), but wit an ultra wide the DOF at f/8 should compensate any misalignment of the adapter.
@nex100,
when using the 12mm Heliar, you crop by crop factor 1.5 already. When I have a 24 MPix camera, I want to use the full potential of the RAW (with slides in the analog world crop was no option anyway
)
With my M9 I have to crop almost every image, as the finder is just not correct enough.
Landscapes with a 12mm for me is not the best solution and you often have to crop the bottom part, because it is useless (and you may get your feet into the picture). I use longer lenses and stitch. With this technique there are no problems in the corners and you get as many resolution and pixels, as you want.
For me the 12mm is best for dynamic scenes or extreme views and positions.
@Carlos, fantastic gear on your home page