The Nikkor-O 2.1cm f/4 lens has always fascinated me. The lens was originally made for the Nikon S-mount rangefinder system, but also found a home on the Nikon F SLR when it was released, but only as a specialty lens that required the reflex mirror to be locked up and the use of an external finder, with focus simply estimated (and of course, no metering!).
The S-mount lens has become extremely rare and sought after now, fetching anywhere from $3,000-$6,000 depending on condition and completeness, while the identical F-mount lens usually sells for under $500. Of course the rangefinder model is a little easier to use with regards to focus, but the selling price is really indicative of the collectors market, not users!
Anyway, I found one for very cheap and snapped it up. Mounting it onto my F was easier than people make it out to be online. The simple breech mount locks into place satisfyingly. I popped my waist-level finder on the F to make it a little more compact, since you can't see through the lens anyway.
The images I've posted above are mostly architectural subjects. The first thing that jumps out at you here is the absolutely perfect straight lines in every direction. The biggest problem with wide-angle lenses on SLRs is their often heinous distortion, often complex and not easily fixed even in software. Even Nikon's most modern 20mm f/2.8 prime has a mustache-like distortion that renders straight lines and architecture very poorly. Due to the superior non-retrofocus design of this lens (necessitating moving the mirror out of the way), this lens seems pretty much free from any distortion - at least none that I could detect. I absolutely love this quality of the lens - I am very sensitive to even a little bit of distortion for one reason or another, and it has been a constant source of annoyance with wide-angle lenses.
Here we see this lens used with human subjects, where it is equally at home, especially when showing the environment as well. Of course you wouldn't be using this kind of lens all the time for people, but still. Of note is the relative lack of fall-off in the corners, even at wider apertures like in the inside photos. While there is certainly some, it's not enough to really worry myself with. I guess if I was shooting chromes with a featureless blue sky, it'd be more apparent, but it would be the same with any wide-angle lens.
Sharpness and detail is simply fantastic. I'm pretty sure it is at least as sharp as the modern 20mm f/2.8 AF-D lens, if not better. Too bad it doesn't fit on my D800E (I tried), that would be an interesting stress-test! This was my first roll with the lens; I'm looking forward to shooting some color film with it this weekend to see how colors render with this lens, but looking at the photos, they seem slightly lower contrast than I'm used to (but, I also developed this roll a little differently than normal, so maybe that's all it is).
Honestly I'm surprised this lens doesn't have more of a cult following. It seems to be an extraordinary performer plagued simply by an unwillingness to use lenses requiring mirror lock-up. Speaking of which - as you can see, focusing is not an issue. Outside, shooting at f/8 or f/11, I simply set the lens to about 10ft. and shot away. At those settings, everything was in focus no matter what. Inside, even wide-open at f/4, there is so much depth-of-field that I didn't miss any shots. In fact, there seems to be no curvature of field, so focus stayed true even in the foreground, unlike many other lenses I've used.
Now not everything is perfect. The finder is like looking through a fisheye lens. The center bulges out like crazy. This doesn't matter in the actual photo, but it makes it really hard to line up photos perfectly straight, at least for me. I often put a 3-way bubble level in the camera's shoe, but of course the viewfinder is taking up that space. Shooting on a tripod would fix that, but I rarely do that with 35mm, so it's a real problem for me. It may not be for others.
The only other issue I noticed is some atrocious blobs of flare on one photo with the sun in it. This lens clearly doesn't like light sources at oblique angles. The blobs aren't even pleasing looking - they look like giant hourglasses. Definitely be careful with bright light in the frame or right outside it.
In summary, if you can put up with its quirks, this is probably simply the best performing prime lens in the 20mm/21mm category for the Nikon F SLR (or F2). In fact, this lens design would probably be a good candidate to revive with the new generation of mirrorless digital cameras (providing they optimize the microlenses for such short focal lengths), such as the newly-announced Sony A7R.
Now what about the Nikon rangefinder system? Well if you happen to own an S-mount Nikkor, I would suggest you check out the Voigtlander 21mm f/4 lens still available at Camera Quest. That's what I use on my Nikon SP and Bessa R2S. But the rendering is definitely a lot different, with more contrast and a more modern feel. But the S-mount version of the Nikkor-O is basically impossible to get, so...
Oh - these photos were taken on T-Max 100 film, rated at 160, and developed in Acufine 1:5 for 40 minutes at 72F. Just for reference.