Wakimoto's signature lens
First, let's look at reports and drawings. The NIKKOR-P.C 10.5cm f/2.5 was designed by noted Nikon designer, Zenji Wakimoto. According to reports, design of the lens was completed in 1949. For approximately two years before that, Wakimoto was continuing design improvements to the NIKKOR-P.C 8.5cm f/2 begun by Saburo Murakami. It is thought that Wakimoto began designing the NIKKOR-P.C 10.5cm f/2.5 after work on NIKKOR-P.C 8.5cm f/2 improvements had been generally completed. These were the circumstances that determined the characteristics of the NIKKOR-P.C 8.5cm f/2 and the NIKKOR-P.C 10.5cm f/2.5. Both lenses were Sonnar-type lenses designed with similar structure and materials. However, the NIKKOR-P.C 10.5cm f/2.5 offered more refined aberration compensation than the NIKKOR-P.C 8.5cm f/2.5. The only tools used in designing lenses at that time were tables of logarithms, abacuses, and perhaps mechanical calculating machines like the Tiger calculator. In a time in which intellect and patience were demanded, Wakimoto called on the "math girls" (see Tale 43) to assist him with the design of a number of noteworthy lenses.
It goes without saying that Wakimoto was clearly a very talented designer.
The mass-production plans for the NIKKOR 10.5cm f/2.5 were released in the spring of 1953. Preparations were completed and the lens was released at the end of the summer of 1954. At the time, the NIKKOR-P.C 10.5cm f/2.5 was the fastest lens in the 100mm class. This lens would become a best seller, proving to be just as popular as the NIKKOR-P.C 8.5cm f/2 previously released in 1948. It became a signature lens with a high volume of both production and sales for the time. It was popular with a wide variety of photographers, from amateurs to professionals and even celebrated photographers including Ken Domon. Even today, more NIKKOR-P.C 10.5cm f/2.5 lenses can be found in circulation than NIKKOR-P.C 8.5cm f/2 lenses. The NIKKOR-P.C 10.5cm f/2.5 appears to be less expensive as well. Though circulation of a large volume of NIKKOR-P.C 10.5cm f/2.5 lenses on the market may have contributed, it is still a very inexpensive lens.
Rendering performance and lens performance
Take a look at the cross-sectional diagram of the lens. It is a typical Sonnar-type lens. It has a simple and rational construction of five elements in three groups. The first group (from left) consists of a single convex lens, the second group of three thick lenses is comprised of two convex lenses and one concave lens, and the third group contains a single convex lens. A primary feature of this type of lens is the arrangement of refracting power asymmetrical to the aperture. While spherical aberration can be sufficiently compensated in Sonnar-type lenses, differences in the shape of spherical aberration depending upon the color are difficult to control, leading to a tendency toward overcompensation of spherical aberration in short wavelengths (blue to purple-blue). In addition, the highly asymmetrical structure often leads to pincushion distortion and lateral chromatic aberration can also be difficult to compensate.
The manner in which the characteristics of these types of aberration are controlled is dependent upon the skill of the designer.
So, how does the NIKKOR-P.C 10.5cm f/2.5 perform? Let's consider both aberration characteristics and results achieved with actual photos.
First we'll look at the design report. A major feature of this lens is the method used to compensate for spherical aberration, coma and astigmatism. Unlike the NIKKOR-P.C 8.5cm f/2, the NIKKOR-P.C 10.5cm f/2.5 under-corrects for spherical aberration. This has the effect of improving background blur characteristics (bokeh). Further, a characteristic of the Sonnar-type lens is the close-range aberration fluctuation, resulting in under-correction of spherical aberration and curvature of field. While it is true that images are not quite as sharp, rendering is gentler and background blur characteristics are more attractive. Therefore, it is thought that this lens is suited to portraits and close-up photography of flowers and the like. Another characteristic of this lens is coma. With the exception of the extreme edges of the frame, some inner coma remains. Although this is a negative in terms of sharpness, elements of flare in high-contrast scenes compress the contrast for improved reproduction of half-tones, especially with film cameras. If we look at a number of Wakimoto's lenses, we will find many with which inner coma remains. It is possible that Wakimoto intentionally designed his lenses in this way. The mystery deepens. Let's continue looking at the aberration characteristics of this lens. There is relatively little curvature of field, and there is a tendency toward under-correction.
The greatest attention was paid to astigmatism. When focusing at infinity, the effects of astigmatism in middle portions of the frame, between the center and peripheries, are slightly greater. I initially thought that this was not very good, but there is a clear reason for it. Astigmatism is eliminated with shooting at distances best suited to portrait photography. Wakimoto clearly achieved another successful design. It seems that he designed the lens with a clear idea of the conditions in which it would be used. Another characteristic that should probably be mentioned is distortion. Though an inherent weakness of the asymmetrical optics of the Sonnar-type lens, distortion was reduced to an incredibly low 0.5%. When we look closely at the work of early lens designers, we clearly see that they had every intention of overcoming inherent weaknesses. I have a new appreciation for this intent that I fully intend to hand down.
Next, let's look at point image formation with a spot diagram.
Point image formation at the center of the frame is good for a clear, sharp image. However, comet-like flare appears in places with lower image height due to the effects of inner coma. Inner coma is cancelled, though, as image height increases. What's more, a slight amount of astigmatism can be seen in the shape of the point formed. Resolution is relatively consistent from the center of the frame to the edges. Slightly more flare occurs with a rather low image height.
Rendering characteristics can be summarized as follows. Resolution is good and consistent from the center of the frame to the edges for sharp image formation. There is, however, a tendency for a slight amount of flare to occur in the center portions of the frame. Further, as points are clearly formed, rendering is more natural. Under-correction of spherical aberration and curvature of field results in beautiful background blur. Users can expect natural and pleasing blur characteristics.
Finally, let's look at some sample photos.
With the exception of the extreme edges, lines are slightly thick but the resolution, contrast and image formation is good throughout most of the frame at f/2.5 maximum aperture. There is not as much flare as might be expected, and contrast compression optimal for black-and-white film is also good. Moreover, there is no unsightly flow in image formation or breakdown in rendering. There seems to be no deviation from design values for resolution or sharpness. When the aperture is stopped down to f/2.8 to f/4, sharpness increases near the center of the frame for even higher resolution.
The area of maximum sharpness expands for sharp image formation all the way to the four corners. The aperture value with the best balance between image formation characteristics and texture rendering seems to also be the optimal aperture value for portraits. When the aperture is stopped down to f/5.6 to f/8, resolution throughout the frame increases for consistently good image quality overall. The level of contrast is also just right, with images that are rich in tonal gradation, as opposed to exhibiting a stark two-tone contrast, which may be partly due in some measure to the inner coma. At f/11 to f/32, sharpness gradually decreases throughout the entire frame due to the effects of refraction. If sharpness is the priority, f/5.6 to f/8 is the optimal aperture range. If rendering suited to portraits is desired, shooting at f/2.8 to f/4 will likely achieve the best results.
Similar or different design?
This tale is not about designers. I would like to consider design improvements by focusing on the relationship between the NIKKOR-P.C 10.5cm f/2.5 for S-mount cameras and the NIKKOR AUTO 10.5cm f/2.5 for F-mount cameras.
At the time the F mount appeared, Nippon Kogaku (currently Nikon) had been examining its lineup of interchangeable lenses. What did they plan to do about F-mount NIKKOR lenses? Designers at the time were working with a view toward application of S-mount lenses that had been so well received. The problem was back focus. S-mount cameras do not have the mirror incorporated into so-called single lens reflex cameras. Therefore, optics for S-mount lenses often had a short back focus. The only lenses that could be diverted as-is to use for F-mount cameras were those with a focal length greater than that of the 13.5cm f/3.5. This meant new designs for most wide-angle and normal lenses. However, at that time, the retro focus lenses commonly used for wide-angle lenses were still developing, and the times were not yet ripe for the design of practical ultra wide-angle lenses. So, the designers selected only the ultra wide-angle 2.1cm f/4 lens as a wide-angle interchangeable lens used with the mirror raised. The 8.5cm and 10.5cm lenses had focal lengths that fell between the 2.1cm f/4 and the 13.5cm f/3.5.
Back focus on the 8.5cm was lacking remarkably, so designers gave up on any thoughts of diversion to the F mount. However, if 10.5cm f/2.5 back focus could be increased by 1-1.5mm, it could be converted to the F mount. By reducing the thickness of the last lens element by 1.0mm, Wakimoto was able to preserve back focus and release the 10.5cm f/2.5 for F-mount cameras. The major question is whether or not this change should be considered a design modification. Strictly speaking, as a change of 1.0mm far exceeds the manufacturing margin of error criteria, the design was modified. However, if we focus on whether or not the effects of aberration compensation were changed with the new optical design, there is very little difference in performance due partly to the superiority of the original optics. The amount of change is equal only to the level of pot correction (see Tale 44). In this case, we should consider that only the minimum changes were applied and optics were applied to the design rather than that the design was significantly improved. In other words, the basic design was handed down and only minor changes were implemented. Wakimoto's 10.5cm lens was popular for more than 16 years until the 105mm lens, designed by his favorite student, Yoshiyuki Shimizu, was released as its successor.
We have asked faithful readers of The Thousand and One Nights for their opinions. Some have stated that while they are aware of the advantages of older lenses and would like to try them, they can't find the lenses anywhere. Or, if they do find them, they're very expensive. S-mount lenses are in especially short supply, and those that are available are expensive. However, as the 10.5cm f/2.5 discussed in this tale and the 13.5cm f/3.5 are still rather reasonably priced, users should be able to purchase them. Users who would like to experience the rendering characteristics of the 10.5cm f/2.5 could try the NIKKOR AUTO 10.5cm f/2.5 for F-mount cameras. As aberration balance is basically the same, this lens should allow users to achieve the rendering characteristics of a Sonnar-type lens. However, as the 10.5cm f/2.5 for F-mount cameras does not utilize a rounded aperture, the feel of photos will likely differ when the aperture is stopped down, especially with backlighting or when a light source is included in the frame.
We would also like to talk about newer lenses and other products in future tales to allow readers the opportunity to experience NIKKOR the Thousand and One Nights for themselves.