NIKKOR - The Thousand and One Nights No.29

W-Nikkor·C 2.5cm F4

Tonight, on the twenty-ninth night, I want to talk about the Nikkor 2.5cm f/4 lens at the strong request of a certain regular user of this lens. This pancake design wide-angle lens was conspicuously unique among the compatible lenses for S. What made this lens so attractive even without any focusing ring? Why has it been highly prized by the regular users? Tonight, let us make clear the secrets of the marble-like Nikkor 2.5cm f/4.

by Haruo Sato

I. Topogon and Nippon Kogaku

The Topogon lens, which is rarely seen nowadays, is an East German Zeiss Jena lens developed in 1933. In actuality, however, it originates from the precursor known as Hypergon (2ω = 135°, F22) invented in 1900 by Goerz. The Goerz Hypergon lens is composed of two convex meniscus elements that have the same radius of curvature. Its most striking feature is that it offered what was then the maximum angle of field. In addition, in terms of aberration correction, the distortion, curvature of field, and lateral chromatic aberration were remarkably small. However, the Hypergon lens, as it was composed of only two convex elements, had the fundamental drawback that it could not correct longitudinal chromatic aberration and spherical aberration. Then Zeiss added a pair of concave elements to the Hypergon without disturbing the symmetrical configuration of the lens system, and succeeded in developing the Topogon (2ω = 100°, F6.3). The Topogon was a dark lens, but it was second only to Hypergon in covering power.

At about that time, the world was heading for war. The entire world gradually became caught in the vortex of World War II. Under the circumstances, the Topogon lens was highly valued for its low distortion and curvature of field, and was then saddled with heavy responsibility as an optical system for aerial photography and cartography. Optical designers in many countries began to devote all their energies to develop a distortion-free, wide-angle lens of the Topogon type. In those days, the lack of an accurate map of enemy territories meant certain defeat. At that time, Nippon Kogaku was also reportedly studying vigorously the Topogon-type lenses. After peace returned to the world, the efforts of the pioneers bore fruit in the world of the artistic culture of photography. Their efforts were incorporated also in the Nikkor 2.5cm f/4.

II. Progress of development

Now let's trace the progress of development of the Nikkor 2.5cm f/4. The optical design was entrusted to Mr. Hideo Azuma, whom I introduced to you on the third night. The optical design was completed in the fall of 1953. Then, a working trial lens was manufactured and the drawings for mass production were released in September 1954. I was surprised when I examined the drawings in detail. First of all, the two concave elements are only 0.45mm thick. The radius of curvature is almost hemispherical, and the difference in the curvature radius between front and rear is very small. In addition, there is a note on the drawings showing the decentering of the lens should be within a tolerance of 60 seconds. The processing of the lens elements must have been extremely difficult. Downsizing is a very difficult process. Proportional reduction of size to one tenth involves tighter tolerances to 1/10 or ten-time higher accuracy than normal.

Regarding this lens system, it is reported that the manufacturing department had a tough time not only with processing but also with fabrication. Handling the flimsy concave lenses was very troublesome. An air blowing gun (spray) could not be used for dust removal. Alcohol could not be used to clean the lenses. The lens could not be held with excessive force. Air blowing with a gun (spray) could cause it to break or fly away. Cleaning the lens with alcohol could cause it to break due to the heat difference caused by evaporation. So, reportedly there was a dedicated cleaning equipment made from feathers for the lens. The lens was really very troublesome on the shop floor. The workers in the production area overcame the hardships and concentrated on implementing the lens, feeling a glow of pride and a sense of mission at creating the Nikkor.

III. Lens performance and imaging characteristics

Please take a look at the cross-sectional view. The Topogon-type lens is configured with four elements, convex, concave, concave, and convex, from the left. At the center of the round lens system, there is a diaphragm to provide a symmetrical configuration. The Topogon-type lens may be considered to offer a triplet design, different from the Hypergon, when it is assumed configured with the elements, convex (concave, concave) and convex. This means that the lens is capable of correcting SEIDEL's five aberrations and two chromatic aberrations.

Let's check what the Nikkor 2.5cm f/4 can do based on both aspects, aberration characteristics and actual shots.

As mentioned earlier, the biggest advantage of this Topogon-type lens is that distortion and curvature of field are minimal.

The individual lens elements have meniscus profiles. This is based on a design idea to prevent the possible occurrence of significant aberration due to off-axis light rays (marginal rays). However, for off-axis light rays, the aberration induced in the front and rear convex elements is not fully corrected by the central concave elements. Consequently, even in the Nikkor 2.5cm f/4, at each angle of field, an insufficiently corrected coma remains. In addition, the meridian coma is seen distributed evenly in a relatively symmetrical pattern. Moreover, the degree of flexibility in correcting the axial aberration is low, and some spherical aberration and longitudinal chromatic aberration also remain.

After examining the lens in terms of aberration, I once had the impression that even the Nikkor lens would not provide satisfactory imaging characteristics as long as it is a Topogon-type lens. However, I noticed that, based on the actual shots, I made an incorrect assumption. Indeed, due to the spherical aberration and insufficiently corrected coma, there is some residual flare in proximity to the closest point at the maximum aperture, and the actual shots offer a soft impression rather than unsharp. However, moderate resolution is yielded with well-balanced (natural) contrast and perspective. I reviewed the design data once again. I focused on the spot diagram, which serves as a means for evaluating the optical image quality of a point object. The object points through this lens appeared as complex deformed images. However, there was a common feature among the images, that is, a large flare encircles the central part of the object points. This was the key to yield moderate resolution and well-balanced contrast. The flare almost disappears at f/8, the aperture frequently used by the lens users, and then proper tone is maintained while sharpness is increased. In addition, the fact that the currently available color positive films and color photographic paper provide rather hard gradation may be one of the reasons that satisfactory results are obtained.

It is well known that Topogon-type lenses reduce the brightness of the edges of image field. This results from the fact that Topogon-type lenses have no way of increasing the aperture efficiency (vignetting factor) to 100% or higher. However, the Nikkor 2.5cm ensures a brightness of the edge of the image field as high as approx. 13% even on the outermost periphery. This value is sufficient for Topogon-type lenses. This teaches us how Mr. Azuma was particular about the lens and he did his work with all his heart for the development of the lens.

Now, let's examine the imaging characteristics at different apertures based on the actual shots.

As mentioned above, at full-open aperture, satisfactory sharpness is achieved in the central part of the image field, although the image becomes gradually unsharp as it approaches the edge. In addition, there is a little bit of flare observed over the entire image field, which leads to the reduction of any excessive degree of contrast and yields moderate tone reproduction. However, a moderate resolution is achieved, there is no excessive perspective, and very natural-looking imaging is attained.

When stopping down the lens to f/5.6, the flare gradually disappears from the central part, and the peripheral image is also improved. When additionally stopping down the lens to f/8, both sufficient resolving power and well-balanced contrast are obtained up to the periphery. Further stopping down to f/11 results in additionally enhanced sharpness, providing the best possible image quality for this lens. When stopping down the lens more to f/16 to f/22, the effects of light diffraction appear, and resolution becomes gradually degraded.

Let's examine the imaging characteristics of this lens based on the sample photos. Sample 1 is a long-range photo. In this shot, I selected my composition to help identify the imaging characteristics of a wide-angle lens. But the actual shot looks so natural as if taken with a standard lens. Sample 2 shows a subject I selected to examine the resolution of the lens. Lines appear fine enough, contrast is not excessive, gradation is rich, and you can enjoy a natural representation.

In this examination, I felt that I could understand why this lens is highly valued by regular users. I suppose the appeal of this lens is the impression that the lens offers imaging characteristics different from those one might expect from a wide-angle lens.

This examination has made me gain a new understanding of Topogon-type lens. I express my appreciation to all regular readers of this column who have requested that I take up this lens.

Relocation of workplaces

Today, I'll tell you about my failures. Although this holds true for any company, we used to change our places of work a lot. Each time when we moved to another place of work, we sorted out our possessions and disposed of unnecessary things.

This is a story about a time shortly after I had joined the company. I was handed two cardboard boxes filled with what looked like junk by my superior and was instructed to sort out the contents and throw away any unnecessary things. Looking into the boxes, I found broken old lenses, hardware, prisms, packages and so on. As I was also a Nikon fan, I salvaged what I believed to be valuables before disposing of the unnecessary items, with a gleam in my eyes just like a treasure hunter. I was amazed when I picked up the HANZA Canon lenses and Hermes. Needless to say, I arranged to properly maintain the valuables.

I saved what looked like finished products, and I disposed of what appeared to be just debris or remains of trial manufactured component parts. Later, I found out that I had thrown away the hood for a 2.5cm f/4. At that time, the utterly scratched up black ring seemed to be worthless. So, I tossed it away into the trash without the slightest hesitation. Later I was surprised to hear its market value. It's no use crying over spilt milk! I apologized timidly to my superior for disposing of such a valuable item, but he smiled at me without concern about the matter. I felt quite relieved. However, when I told the whole story to a friend of mine (Nikon fan) at one point, I was thoroughly scolded. Enthusiasts are much more strict.