so my question is the same...
What is the correct configuration of the Subal ports for nikon D800 or D4 with sigma 15mm?
This has already be mentioned (by fletcher and by me):
If you want to use the Sigma with a minidome (like the 4 inch DP-100 from Subal which is the same like the one from ZEN) you must shave the lens shades and use it without an extension ring. Without shaving the lens shades it will physical not fit into the small dome (the lens shades will come physically into conflict with the inner side of the glass). With a 15mm extension ring as Subal indicates on its website (and did the same also to me when I specifically asked) it would fit into said dome; but this combo probably would not work at all since then the angle of view would be even more "cropped" than in my case (any extension ring puts the glass of the dome further in front and thus the 180 degree field of view of the lens will be more covered). There is plenty of information on shaving the Sigma lens in the net, make a google search!
If you use the Sigma with a large dome ports like Subal's DP-FE it will work without an extension ring as well. This is for "regular" 180 degrees fisheye usage and less for CFWA (because the glass of the large dome ports puts the captured objects futher away than the possible closest focus distance of the lens). Subal indicates this as well: DP-FE no extension ring!
hope this makes it clear now.