In the box along with the Sony NEX-7 my friend sent a Nikon Nikkor 20mm f/3.5 UD. I was surprised, pleased, and, of course, very interested to see how it performed.
Historically, Nikon had earlier designs for a 2,1cm lens. Those were symmetrical and the rear element set recessed deeply into, first, Nikon rangefinder camera bodies and later the Nikon F SLR. In the case of the SLR the mirror had to be mounted up and out of the way so the rear element set could be properly positioned, thus nullifying the benefits of being able to look through the lens.
Thinking about it for a moment, the 20mm f/3.5 UD I was now holding represents Nikon's first strongly asymmetrical ultra-wide angle lens designed specifically for the Nikon F SLR. For its time the lens would've been rather unique.
Off to beers with a friend one day I took one of my trusty Sony NEX-5T cameras (I have far too many of these because, well, they're cheap now) with a Lens Turbo II focal reducer and mounted the old Nikkor. The first thing I noticed was just how large the 20mm Nikkor is when used on a very slim, very small APS-C Sony mirrorless camera. The next interesting thing I would notice had to wait until I returned from the pub.
One of the images that I'd taken had deliberately included sections of strong daylight highlights and deep pub-interior shadows. The (now) small 16mpixel sensor is well known for it's long 13EV dynamic range. The newer 24mpixel APS-C Sony sensors only slightly extend the range to 13.4EV (NEX-7). So this, to me, means the old sensor will continue to perform very nicely for much of the kinds of photography I tend to do.
Liking fields of subtle grays I am pleasantly surprised by the detail and "creaminess" of the image I took of my friend. As you can see, there is detail deep into the shadows and the highlights roll off nicely, just like when using old silver halide film.
The lens appears to produce little to no flare, which is quite remarkable considering the age of the optic and the fact it is only single coated. It is sharp from wide open.
I think this lens is a "keeper."