Vivitar 20mm f3.8
My test lens was a Nikon Non-Ai mount Vivitar made by Kino. It is a solid, heavy lens with a large front element. My copy had dust in it and some separation in the rear element group. I also noticed possible oil spots in the grouping.
The lens is reasonably sharp, even wide open, even with the issues. However, that sharpness is damaged by ghosting, diffusion and flare in high contrast scenes. Since my copy wasn't perfect I expected minor issues and I assume a cleaner copy would fair much better. With such a small rear element any blemish towards the rear of the lens becomes a factor.
The lens does perform well in close ups. Very well, in fact. Close focus is less than 6 inches and the focus throw is very long, allowing you to get critical focus of near objects quite easily. Bokeh is surprisingly smooth. With it's wide field of view you can create unusual and striking compositions. Use this lens in the garden.
Focusing in normal ranges is almost impossible, though. While it takes 2 whole turns to go from 6" to 10 feet, it only takes a nudge to go from 10 feet to infinity. That's right, a nudge.
Fringing is noticeable at 50% view but tightly contained and easily corrected. Most ultra wide lenses suffer from fringing. Avoid white backgrounds. Bokeh fringing is minor, though.
It's no Flektogon, that's for sure, but it can produce decent images in the right situations.
f3.8 - 75% ish crop - Approx 10in away