It shared popularity with the NIKKOR-P 8.5cm f/2 lens released in 1948, and established itself as a best-selling lens. Offering superior optical performance from the initial design, the optical systems developed for this lens was continued on into the Nikon F era. The initial design used Sonnar type optics (5 lenses, 3 groups). As you can see from the cross-section in Fig. 1, the thick lens included a group made of three lenses cemented together, and offering sharp, solid images. This optical system was continued for about 17 years until the 1970s, thanks to the accurate design concepts and superior optics design implementation.
In 1971 the lens underwent fundamental design changes, emerging as the Nikkor Auto 105mm f/2.5. The optics were designed by SHIMIZU, Yoshiyuki, who was one of WAKIMOTO's disciples. He designed a large number of lenses from the early Nikkor Auto through AI Nikkor, and was also active in designing optics for diverse other applications, including object lenses for microscopes. He was active as a designer until quite recently, and still comes to Nikon as an educator, and he has taught me much since I first entered Nikon. In fact, he is probably one of the most experienced people at the company.
The design for the 105mm f/2.5 was completed in the winter of 1966. The design proposal for this best-selling mid-range telescopic lens was continued for three generations, from the Nikkor Auto to the New NIKKOR, the AI Nikkor and then to the AI-S Nikkor. And surprisingly enough, the AI Nikkor 105mm f/2.5S being sold now still uses the same basic design: a design on the market for 34 years. We will enter the 21st century in about half a year, and the superior performance of this lens is proved by the fact that the basic design required no changes over 34 years.
Lens structure and features
Take a look at the cross-section of the AI Nikkor 105mm f/2.5 lens (Fig. 2.). This design is based on the Xenotar-type lens (5 lenses, 4 groups) with thick, convex lenses, rather than the Sonnar type with its many asymmetric components. From the left, there is a convex lens, a cemented lens consisting of a very thick convex lens and a concave lens, and then a convex and concave pair after the stop.
Compared to the previous model with Sonnar type lens construction, it offers significant improvements in close-range aberration fluctuation, as well as peripheral light, spherical aberration and coma. In particular, it delivers a beautiful balance of focused and defocused (blurred) images, as well as higer resolution with natural gradation. The Xenotar-type lens design with the ideal aberration correction made it the perfect lens for portraits.
An anecdote back from the era of the Nikkor Auto lenses. There was a lens that delivered incredibly sharp resolution, but the defocus image (blur) was not so good. particular lens branded the whole Nikkor Auto family with a reputation of having bad defocus images, especially in Japan.
As the Nikkor 105mm f/2.5 demonstrates, however, at the same time Nippon Kogaku K.K. was also making lenses offering the perfect combination of defocus image, tonal rendition and gradation.
While the new and old lenses had differences in front and rear lens diameters, overall length and thickness are about the same, amazingly enough. Inside the lens barrel, in fact, it is hard to tell them apart. At about that time a number of NIKKOR lenses were upgraded with new optics, taking care to maintain the same familiar appearance, size and feel. Considering it is hard enough to improve optical performance, the designers must have faced considerable challenges achieving their targets in spite of tight restrictions on total length and diameter.
Rendition characteristics and lens performance
So how is the rendition of the AI Nikkor 105mm f/2.5 lens?
As I have said before, evaluations are always subjective, so please consider my comments as my personal opinions, for reference.
As mentioned above, this lens has the features of a symmetric design, and in particular distortion is extremely low. The image is flat to the periphery, and astigmatism is very small.
The lens also has characteristics of spherical aberration and coma. Basically close-range aberration variation is small, but at portrait distances the correction for aberration seems to be slightly insufficient. The insufficiency as far as spherical aberration in particular is what makes defocus background appeared beautiful. The aberration balance has been calculated carefully for use in portraits. When the aperture is open contrast is good, and delineation is soft.
Examples 1. through 4. were taken at f/2.5.
Examples 1. and 2. show the subject relatively close to the background. In Example 1., the background has partial back-lighting with the sun through the trees, making for conditions generally susceptible to double-line effect, ghosts and flare. The sharpness with the focal plane is excellent, with no optical flare. There are no obvious flares or ghosts at all, and in particular the defocused background image is superb. Even under these conditions there was no double-line effect, and the tonal gradation of the background remains excellent.
Examples 3 and 4 are close-ups.
Example 3 was taken under the worst possible conditions, with the sunlight shining into the lens, but even so image quality deterioration due to flare is minimal. Tonal rendition is excellent, and background blurring beautiful. There is a softness within the sharp focal plane, with delicate renditions. The result clearly shows the aberration correction characteristics, and the excellence of the coating.
The AI Nikkor 105mm f/2.5S now available uses a newly-developed multicoating (Nikon Super Integrated Coating, or NSIC) for even better color balance.
I would like to briefly run over the performance at various f-stops.
Rendition from f/2.5 to f/4 is as I have just described. I might add that there is very little vignetting, and that optical characteristics are quite uniform across the entire image. Most portraits will be taken between about f/2.8 and f/4.
Between f/5.6 and f/8 sharpness improves even more, and superb delineation is seen across the entire image. I think this would be ideal for outdoor snapshots and scenery.
The same trend exists from f/11 to f/22, although contrast increases at this level.
Mr. SHIMIZU likes to go fishing or play mahjong... here are a few interesting episodes to amuse you.
When I was still new to Nikon we had our regular (unofficial) mahjong tournament in the department, and he revealed a totally different face. He was sharp and fast, a different person entirely. One of the other people explained that "the best optics designers are good at mahjongg, because they combine flashes of sheer genius with the objective ability to observe and analyze." I had to agree.
When I had the occasion to check later, I was astonished to discover that most of the Nikkor lenses loved in my student days were designed by him. As he taught me their aberration and other characteristics, he also told me tales of how difficult many of them were. I once asked him a geometry optics question, and he pulled out a stack of ancient notebooks. "I had the same question, and solved it. Here, take them." These "SHIMIZU Notebooks" were almost legendary in the department. Before they had all these modern optics design tools, they knew their basic much more solidly then. It was this hard-won expertise that made Nikkor what it is today.