No 12mm rectilinear FF lens?

On 2014-11-27 10:33:06 +0000, Me said:

Not really because with the APS-C format you can design the rear element
of a lens closer to the image plane, which turns out to be pretty
crucial with super-wide lenses. A full frame SLR's mirror would
probably smash itself into pieces against the Sigma lens you mention.

That's actually not correct, as the Sigma 8-16 will mount on any Nikon
FX SLR body. The mirror will not touch the rear of the lens. It will
however vignette.

I'd rather have you try this on your camera.

If it's so /crucial/ to get the lens rear element close to the image
plane, then how come the best performing ultra-wide lenses are for SLR
formats?

They aren't. The best performing ultra-wide lenses are for the
rangefinder format. Followed by cine.