Apparently, the Venus Optics Laowa 7.5mm f/2 is a great little UW. It's got a few issues with flare, distortion and vignetting, but it's super compact, all metal construction, it's got an aperture ring (full stop increments only?), and it's only $500. The 105mm f/2 turned out to be a sensational lens when tested by lenstip, but some issues with quality control and its high price led them to have some reservations. At half the price of currently available ultra wide zooms, 1/2 to 1/3 the weight, and with a good full stop advantage in light gathering ability, I think the Laowa 7.5mm f/2 might be the one to get.