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  6. Tale 9 : Nikkor 13 mm f/5.6

The world's widest angle of field Tale 9 : Nikkor 13 mm f/5.6

In this 9th Tale, I introduce an ultra wideangle SLR lens for F mount, "Nikkor 13 mm f/5.6" that has the world's widest angle of field (2 omega = 118 degrees) among ultra wideangle lenses of the ordinary projection system for 35 mm (135) format SLR cameras.
This record is not yet renewed even at this moment of 21st century.

Now we follow the changes of 13 mm f/5.6.
Nikkor 13 mm f/5.6 was released (on order basis) in March 1976. It appeared one of so-called "new Nikkor", the lens with the new outlook design, multi-layer coating and automatic stop.

I've heard that it won highest praise as "it has extra wide angle of field, and that, excellent performance", since it can obtain very sharp focus over the entire shooting distance from the infinity to the closest 0.3 m due to the adoption of "collection of close distance aberration (Nikon-special floating system) ".
Then modification of the AI coupled exposure system (automatic correction from fully-open f-stop) was applied to this "Nikkor 13 mm f/5.6" and it appeared again as "AI Nikkor 13 mm f/5.6" in June 1977.
Further in March 1982, it was changed to "AI Nikkor 13 mm f/5.6S". After that, the basic design was not changed until it was discontinued and it had been selling long, for more than 20 years.

The optical system of "Nikkor 13 mm f/5.6" was designed by Mr. MORI, Ikuo of 1st Optical Section, Optical Designing Department (then).
Mr. Mori was the right-hand man of Mr. WAKIMOTO, introduced in Tale One. The lenses designed by Mr. MORI varies from ultra wideangle Nikkors for F mount, PC Nikkors, Nikkors for Bronica, Nikkors for large format cameras to EL Nikkors for enlarging.
I have been working with him till a recent date. He was a frank and gentle person. We were always astonished at his knowledge and insight of lens designing. I introduce some of the anecdotes about his design handed down to this date.
One is a story of when he renewed the old "EL Nikkor 50 mm f/2.8" (designed by Mr. WAKIMOTO). The wizard lens designer Mr. WAKIMOTO said, "I take off my hat to Mr. MORI!" as Mr. MORI made the renewal design perfectly well.

EL-NIKKOR 50mm f/2.8
EL-NIKKOR 50mm f/2.8N

Of course, the old "EL Nikkor 50 mm f/2.8" was an excellent lens and Mr. WAKIMOTO invented this WAKIMOTO type (EL-Nikkor type) lens with the hard work.
Mr. WAKIMOTO had no other word than "God, it's a knockout!" to Mr. MORI who made it much better than expected, though it was made at a later time when everything was advancing than before.
Another story. Mr. SHIMIZU, the originator of Nikkor Auto who was introduced in Tale 5 mentioned in retrospect, "I thought it's impossible to design that ultra wideangle 13 mm. Mr. MORI was a super designer."
Mr. MORI full of such anecdotes retired a few years ago at the retirement age.

From left, "AI 20 mm f/3.5", "AI 18 mm f/3.5" and "Nikon F3", "AI 15 mm f/3.5", "AI 13 mm f/5.6"

Japanese master designers are seldom known to the public, but their course of the work can be followed via their reports, log of development, notes, patents, and so on.
It tends to be thought that "13 mm f/5.6" was designed in order to make the angle of field of "Auto NIKKOR 15 mm f/5.6" (1973) or "NIKKOR 18 mm f/4" (1975) wider, but it is not true.
Mr. MORI started to design ultra wideangle lenses with various specifications in parallel before 1970, with setting such conditions as focal lengths of 13 mm, 15 mm and 18 mm, and apertures of f/3.5 ~ f/8. In his reports of those days, original types of each lens were shown (to our surprise, he was even studying the lens with an aspherical lens).
Out of them, "15 mm f/5.6", "18 mm f/4" and "13 mm f/5.6", in that order, were selected for production models, after a few times of prototyping.

Now, let's follow his history of development of "13 mm" more in detail.
First, he proceeded the development with specifications of "13 mm f/8", finished the design in 1971 and issued the blueprint in March of the same year. Details after that is not known, but "13 mm f/8" was never produced.
The design was further improved and faster "13 mm f/5.6" was prototyped in 1973 and it was released (on order basis) in March 1976.
Mr. MORI applied the patent for the invention of this ultra wideangle lens to U.S. Patent Office in 1971 and was granted the U.S.P. (patent) in 1973. This ultra wideangle lens was judged as an invention of a new type lens.
That was the time when a large scale computer was introduced to the lens designing, but its performance was much inferior than that of a current personal computer.
It took too much time and the simulation was not sufficiently made. His contribution to Nikkor lenses was immeasurably great.

1. Lens structure and features

(Fig. 1)

Please excuse me if the following story is a little difficult.
First, take a look at the cross-sectional view of "Nikkor 13 mm f/5.6" (Fig. 1). You will be surprised at the huge front lens (maximum diameter of the lens:115 mm) and the complicated configuration (12 groups, 16 elements).
However, this complicated optical system is also based on the same retrofocus type as the ordinary SLR wideangle lens.

Facing the drawing, the front part (left) ahead the aperture stop (indicated by an arrow in (Fig. 1)) forms a concave lens (group having a negative refractive power) on a whole, and the rear part (right) behind the stop forms a convex lens (group having a positive refractive power).
The lens group behind the stop is a master lens developed from so-called Triplet and Tessar type.
And the lens group ahead of the stop might be easily understood if it is regarded as a wideangle converter to reduce the image size.
Why are such a large number of lenses needed?
The fact is that number of lens groups ahead of the stop are needed to correct the distortion of the master lens without increase of aberration in close distance. If compared with the diagonal type fisheye lens "Fisheye Nikkor 16 mm f/2.8", it is quite obvious that the design of ultra wideangle lenses means , in a sense, "the battle with distortion".
What is the feature of aberration of "13 mm f/5.6"?
This lens has the structure of extreme retrofocus type that keeps the backfocus of more than three times of its focal length (13 mm). Generally speaking, it is usually difficult to correct distortion, lateral chromatic aberration and increase of aberration in close distance in particular with ultra wideangle lenses. However, "Nikkor 13 mm f/5.6" has the distinctive feature of little distortion and little lateral chromatic aberration except in extreme periphery, although it has extremely asymmetrical power arrangement.
Astigmatism and curvature of field are well corrected and increase of aberration in close distance that is one of the weak points of retrofocus lens is also well controlled due to the Nikon-special floating system.

2. Lens performance and imaging characteristics

Nikon F2, Nikkor 13 mm f/5.6, f/5.6, 15 sec. RVP
(C)2000 SATO, Haruo

What image quality does the "Nikkor 13 mm f/5.6" give?
Please understand that the evaluation is strongly affected by each person's subjective point of view. Please refer to it only for your reference.
I wish to comment on performance at various apertures, using sample shots.

At the full aperture (f/5.6), it has excellent definition and contrast, except in the extreme periphery. However, with such extreme ultra wideangle as this, some vignetting can't be avoided. But brightness of the edge of image field is quite abundant, far better than symmetrical type ultra wideangle lens.
(Sample shot 1.) is the night view of the cherry blossoms. It is easily noticed that it has sufficient sharpness. No deformation of light sources proves that sagittal coma flare caused by coma is very low.

Nikon F2, Nikkor 13 mm f/5.6, f/8, 1/500 sec. RVP
(C)2000 SATO, Haruo

At f/8~11, vignetting is improved and the performance at the periphery is enhanced, the image becomes sharper.
(Sample shot 2.) is taken at f/8. It is noticed that the brightness of the edge of image field is sufficient enough at the extreme periphery.
At f/16~22, sharpness becomes more uniform over the entire image field, but the resolving power decreases a little due to the influence of diffraction.

As a whole, it gives a sharp image even at the full aperture, since its full aperture f-number is rather slow of f/5.6 and the brightness of the edge of image field is sufficient. Therefore, the difference of sharpness by stopping down is relatively small with this lens. It had better to consider that the aim of selecting the aperture stop is to control the brightness and depth of field.

Ghost images are fewer than ultra wideangle of the same class, but some ghost images of the shape of stop blade are formed. The formation of ghost image, in a sense, is fatal with an ultra wideangle lens. However, when there is no light source, it can be relatively easily eliminated by swinging the image field a little to left or right, or by halation cut (photographic technique of shielding the harmful light entering into the lens by a tool like a black board or by hand, checking with the viewfinder if there is halation or not).

Mr. MORI, Ikuo

The remarkable achievement of Mr. MORI, as a designer, is just as described in the text.
Apart from the work, he is a thin, gentle looking and moderate gentleman. He likes fishing very much and we've often got a share of his catch. He was a big senior who was kind to his juniors and relied on by them.
However, he has another face. To tell the truth, he loved drinking sake (Japanese rice liquor) very much (maybe a heavy drinker). He had many episodes relating to drinking. Let's introduce one of them. It's a story of three cigarettes of Mr. MORI.

A good while ago, he was drinking sake together with his colleagues. The more Mr. MORI drank, the more he was getting cheerful and talkative, and he smoked cigarettes more often, since he was a heavy smoker, too. A drink, a smoke, . . . a drink and a smoke . . . . He lit a cigarette and puff at it and laid it on an ashtray. When he devoted himself to conversation, he used to take a swallow of his sake. Then he lit another new cigarette and tossed off his glass. Again he lit a new cigarette . . . . At last there were three cigarettes on the ashtray and smoke was rising. When he was just to take fourth cigarette, someone said to him, "Mr. MORI, there are already three cigarettes lit !"
Then Mr. MORI said, "Gosh, I have made a mess of it !" and he took one cigarette in his mouth and held other two in his both hands and smoked them in turn one by one. Everybody laughed at him.
Mr. MORI was a man of fortitude and vigor and, at the same time, full of sense of humor.

Note

This issue first appeared in "NIKKOR Club Quarterly" magazine; No. 175 (2000-12-31), published by the NIKKOR Club, and was revised for Nikon's webpage.
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