Symmetrical wideangle lens that has been passed on to this day Tale 1 : NIKKOR-O 2.1cm f/4
1. NIKKOR-O 2.1cm f/4 lens -- construction and features
This ultrawideangle lens, which is now attracting attention in the used lens market, was originally the S-mount interchangeable lens for the Nikon S-series rangefinder cameras launched in 1959.
Thereafter, it went on sale as a interchangeable lens for the new Nikon F SLR in December of the same year.
The most obvious characteristic of this lens, as can be seen from the photograph, is that it projects a long way back from the mount. This leaves a gap of only 7 mm between the lens and the film plane.
For this reason, it could only be used on a manual-focus SLR with a function for locking up the mirror, in practice the Nikon F, F2 series, etc.
For framing, the dedicated viewfinder fits in the accessory shoe of the F or F2, and focusing is carried out manually with distance scale.
The optical configuration of this lens is somewhat complicated: it is a symmetrical"concave-convexo-concave" system as shown in (( Fig. 1.)).
This is very different from the retrofocus type of wideangle lens, with its long back focal distance, which is the mainstream in current interchangeable lenses for SLR cameras.
Basically this symmetrical "concave-convexo-concave" lens configuration was first adopted in the well-known Biogon lens (( Fig. 2.)), but the same symmetrical "concave-convexo-concave" lens configuration that appeared in the Nikkor-O 2.1cm f/4 lens was an invention of the distinguished Nippon Kogaku K.K. (now Nikon Corporation) designer, Mr.WAKIMOTO, Zenji.
Mr. WAKIMOTO, was one of the designers of the S and F series Nikkor lenses, and the creator of the Micro-Nikkor lenses (lenses for precise duplication).
He was rewarded with the "Emperor's Purple Ribbon Medal" for his work in developing the Ultramicro-Nikkor lenses (lenses for imaging in IC (Integrated Circuit) fabrication).
To return to our theme, the inventive feature of the design of the Nikkor-O 2.1cm is its eight(8) elements in four(4) groups, with the two inner three(3)-element groups sandwiching the iris (stop).
The "Biogon type" lens has three(3) elements, concave, convex, and concave, in that order cemented together, whereas the "WAKIMOTO type" (if I may dub it thus) has elements in the order convex, concave, and convex.
If you're smart you will have spotted that the "WAKIMOTO type" is better for making a larger aperture.
It may not be immediately obvious, but this three-element group acts as a convex lens.
Although it is convex overall, the "Biogon" has a concavo-convexo-concave structure, and thus a majority of concave lenses. This means that the central convex lens must have a high power, and is thus a very fat little lens, increasing the aberration.
However, the "WAKIMOTO" lens is "convexe-concave-convex", with a majority of convex elements, which reduces the aberration created by the surfaces. Moreover, since this lens group is close to the iris, it has the largest effect on brightness.
Thus whereas the Biogon 38mm (for 6x6 format) and Biogon 2.1cm (for 35mm (135) format) had apertures of f/4.5, the Nikkor-O 2.1cm at that time was the fastest lens in its class, with an aperture of f/4.
Actually this "WAKIMOTO" lens principle is still being applied, both at Nikon and elsewhere. Of course, there have been great advances in design technology, but the idea is still full of strength. Among current Nikkor products, the lenses for large-format cameras are just such examples.
(( Fig. 3.)) shows the configuration of the Nikkor-SW 65mm f/4 large-format camera lens, a top-class fast wideangle lens in the Nikkor-SW series. As you can see, this is a "Wakimoto" lens, as are also the Nikkor-SW 75mm f/4.5 and Nikkor-SW 90mm f/4.5.
Good inventions usually age well, and for over twenty years this idea has gone from strength to strength, with the ensuing developments a tribute to its designer.
To put them in reverse order, two characteristics of a symmetrical wideangle lens design are (1) the small distortion and (2) a tendency to vignetting.
The second of these, vignetting, can be cured by stopping down the lens, or by dodging at the printing stage. Distortion, on the other hand, is an intrinsic property of the image, and is therefore very difficult to correct.
The presence or not of distortion greatly affects the finished image. For example, for landscape photography with a wideangle lens of small distortion, mountains and seascapes without great depth in the picture appear so natural that the lens angle may not be obvious.
On the other hand, in pictures which include a lot of depth, such interiors or close-ups in a group of people, a certain degree of distortion may actually add realism to the image. But a wideangle lens with small distortion can tend to give a stronger perspective impression.
Since it is easy to assess the degree of distortion, once you see this as a characteristic of each of your own lenses, you can start using it as a criterion when choosing a lens for a particular subject. (See the example photograph.)
2. Lens performance and imaging characteristics
So what image quality does the NIKKOR-O 2.1cm f/4 give ?
To discuss the meaning of "image quality" in depth would take considerable space, and involve a preliminary understanding of some of the technical terms involved. Thus it is not my intention to go into this here. It will be rather abstract and generalized, but I will just touch on the principles of image quality.
As we have already said, the 2.1cm f/4 has almost symmetrical front and rear groups sandwiching the iris. In particular, for wideangle lenses other than for SLRs, the most common way of achieving the wide picture angle is through the use of concave lens groups at front and rear, in a "concave-convexo-concave" configuration (the actual lens configuration is "concave - convex - stop - convex - concave").
As mentioned already, the important characteristics of this type of lens are: (1) low distortion, and (2) a tendency to vignetting.
These tendencies are present in the NIKKOR-O 2.1cm f/4, but there is also a third characteristic : (3) small lateral chromatic aberration
Lateral chromatic aberration is a form of aberration which gives colored fringes on the periphery of an image, but which is not corrected by a smaller aperture. Low lateral chromatic aberration is an essential condition for a high-resolution lens.
On the other hand, this lens was designed in the days of limited computing power, and exhibits some residual inner coma aberration, slightly low contrast, and slight loss of resolution at the periphery.
Let's look at the characteristics at different apertures.
- With the iris fully open at f/4, the resolving power is relatively good, but the peripheral contrast is slightly low, and the overall image is somewhat soft.
There is some vignetting, which may be a problem for transparencies.
But since culvature of field and astigmatism are very little, there are no obvious defects.
Also, since the contrast is suitably compressed, good gradations can be obtained, yielding black-and-white prints with no special corrections.
- Apertures from f/5.6 to f/8 cure the vignetting problem. The contrast also increases, and the resolution is even better near the center of the image. Even so the contrast is not excessive, producing well-rounded images.
- With the iris stopped down to f/11 to f/22, vignetting is completely eliminated, and the resulting images have both contrast and resolution very satisfactory.
This is a personal opinion, but I think the lens produces the very best images at f/11 to f/16.
Even close to the minimum aperture (f/22) images are not over-contrasted. This is one difference from present-day lenses. To give the downside, it has to be said that the residual aberration is high, but once you understand its characteristics, this lens will serve well.
The coating is a single layer, but I have never experienced serious problems with ghosts. The coloring is on the yellowish side, but that may a property of my particular lens.
The balsam has yellowed with age, and I use the lens principally for monochrome work. For color transparencies in particular, the clear images given by a modern design such as the AI AF Nikkor 20mm f/2.8D or AI AF Nikkor 18mm f/2.8D may be preferred.
Apart from the Nikkor-SW and other large format lenses, other Nikon developments of the "WAKIMOTO symmetrical wideangle lens" are the Nikkor 35mm f/2.8 lens on the Nikon 35Ti Quartz Date (1993) and the Nikkor 28mm f/2.8 on the Nikon 28Ti Quartz Date(1994).
The influence of Mr. WAKIMOTO's inventions and the Nikkor design concept continues to this day. For SLR wideangle lenses, the application of aspherical techniques to mass production has led to more compact designs and better specifications, and given us ultrawideangle zooms. Although the objectives and design concepts for imaging are different, basically the inspiration of the creation, nurture, and development of the Nikkor lenses is still there.
Mr. WAKIMOTO, Zenji (1924 - 1996)
Mr. WAKIMOTO, Zenji moved on exalted levels, as a Director, and later as a Consultant, and I never came to know him well, but every time we met I was struck by his willingness to help anyone.
There are many stories about him. Without being able to vouch for its veracity, I will just relate one such story that illustrates a side of his character.
In his younger days, ray-tracing calculations were all done using abacuses and logarithm tables, and there were several dozen young women entrusted with this task. One of Mr. WAKIMOTO's responsibilities was to organize these young women with a taste for figures, who were taking the place of today's electronic computers.
Anyway, one day at lunchtime, Mr. WAKIMOTO took off on his own to the roof of the Ohi Plant --- at the time unfenced, and perhaps with just a "KEEP OUT" notice. Not content with merely being on the roof, he was sitting dangling his legs over the edge and gazing into space.
Far from complying with impassioned requests to come down from a member of the administrative staff, he then proceeded to walk around the very edge of the roof, as though walking the tightrope in a circus. Astonished onlookers apparently rushed up the stairs to bring him in. The story goes that it was after this incident that a fence was erected around the edge of the roof.
He was an intrepid spirit, way beyond the ability of someone with my fear of heights to imitate. I remember hearing this story, as a new member of the company, "Wow, that's the kind of guts and determination you need to become a famous designer !"
Mr. WAKIMOTO produced many clever and wonderful inventions. He is well known in the Japanese industry, though never quite attained the fame of an internationally acclaimed designer.
Nippon Kogaku K.K., and now Nikon Corporation, has produced many great designers, but in my personal view he is perhaps the greatest.
This issue first appeared in "NIKKOR Club Quarterly" magazine, published by the NIKKOR Club, and was revised for Nikon's webpage.
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