I have a 12mm Sigma very wide angle lens in "T" mount. Largest aperture is f8. It stops down to f16. I have been thinking of mounting it on my M39mount Bessa R 35mm rangefinder camera, shown below with a Soviet 35mm lens:
I tested fitted the lens on my Nikon N90S film SLR. Very wide view, of course. Then on my Nikon D70S digital SLR. The view is equal to 18mm on a 35mm film camera. NOTE: The "18mm" on a standard 18-55mm lens for a digital SLR is equal to 27mm to 82.5mm on a film camera.
So, the view with this 12mm lens is a true 18mm field of view. I was trying to get the camera to fire with a flash, it would not do so. Experimenting, I tried sans-flash with various shutter speeds and found 1/4 second doable. Below, see the results, hand-held at 1/4 second shooting in a rough circle.

I'm hoping, but haven't checked yet, that my version of Photoshop Elements has a stitching program so that I can create a 360 degree image.
Thanks for looking!
Scott