NIKKOR - The Thousand and One Nights No.26

Ai Micro Nikkor 55mm f/2.8 (Latter Part)

Tonight's tale is a sequel to Tale 25 about Micro NIKKOR. Micro NIKKOR is one of the most important lenses in the history of development of NIKKOR lenses. Tonight, I'd like to talk about the Ai Micro Nikkor 55mm f/2.8 and the progress of development and imaging characteristics.

by Haruo Sato

I. Development of Micro Nikkor 55mm f/2.8

In those days, the optical system designers in the development program were urged to crack the difficult technical challenge of implementing Micro NIKKOR lenses with higher aperture, and further improving the field-proven quality lenses designed by Mr. Wakimoto. Mr. Yoshinori Hamanishi, assigned to what was then 1st Optical Section, Optical Design Department, resolved the technical challenges. Mr. Hamanishi developed a number of high-quality lenses including the 55mm and 60mm Micro NIKKOR. Mr. Hamanishi was one of those who inherited the zoom lens design technology from Mr. Soichi Nakamura, and he exhibited his superior creativity especially in the design of telephoto-zoom lenses. In fact, he was involved in the design of many popular Ai NIKKOR telephoto-zoom lenses including Ai 80-200mm f/4, 80-200mm f/2.8 ED, and AF 80-200mm f/2.8 ED. He was a brilliant optical designer and he showed his excellence in optical theory, especially the design principles of zoom lenses. When I had just joined the company, I learned one day from Mr. Hamanishi how to determine the solution curve of a zoom lens composed of multiple groups. Mr. Hamanishi put a bunch of reference materials on the desk, saying with a smile: "It would be a good lesson for you (to deepen your understanding) to read these references." I was stunned and utterly surprised and then turned some pages timidly. I found many handwritten numerical formulas that Mr. Hamanishi derived by himself. I tried to follow the formulas to understand their significance. However, the formulas looked too well conceived for a poor student like me to interpret.

The formulas were composed of very well regulated functions including all the confusing numerical values and factors enclosed in parentheses. A brilliant person should have been able to immediately understand the physical significance of the formulas, but, sad to say, I did not have the ability in those days to interpret them. As a result, I had to start with the derivation to understand the significance of the formulas, which caused me to have a stomachache.

Though being strict about his duties, Mr. Hamanishi is openhearted and great fun when not at work. There are many interesting stories about his personal life. Among them, his own method for judging personality based on the blood type was well known in the workplace, and especially his views regarding "the working style of optical designers according to the blood type" carried a lot of weight, and impressed us and made us feel pleasant. I have type O blood and thus I would "design boldly and hearty" according to the Hamanishi theory. Is it true of me? Mr. Hamanishi has been a good teacher and a man of experience for me. He is now responsible for the fostering of designers in the optical design function in a different section. He may be making it a rule to put a bunch of reference materials on the desk of newcomers.

II. Lens Performance and Imaging Characteristics

Now let us discuss the optical system of Ai Micro Nikkor 55mm f/2.8. Take a look at the cross-section view. You can see that Micro Nikkor 55mm f/2.8 is a typical Gaussian lens. This represented a departure from the Xenotar type which had been supporting the long history of Micro NIKKOR. Mr. Hamanishi adopted the Gaussian type because of its advantages of applicability for higher apertures and less close-range aberration fluctuation compared to the Xenotar type. This contributed to the evolution of the f/3.5 lens into f/2.8 and the achievement of enhanced optical performance.

In addition, of special note is the adoption of floating adjustment. Mr. Hamanishi applied the floating adjustment used in the conventional wide-angle and ultra wide-angle lenses to the Gaussian-type micro-lens. The lenses existing then were designed to provide well-balanced corrections for the aberration at infinity and at a magnification of 1/2. In practice, however, even the Xenotar and Gaussian lenses of the symmetry-type, which have greater flexibility in compensating the close-range aberration fluctuations, could not properly accommodate any fluctuations in the curvature of field and coma. Then, the optical designers in each competitor were struggling to stop down the lens f-numbers to ensure a higher degree of flexibility in the design and suppress the close-range aberration fluctuations. However, we at Nikon were urged to select f/2.8 for the next Micro NIKKOR. This caused us to choose the correct solutions. As a result, Mr. Hamanishi created the Gaussian-type micro-lens equipped with the floating adjustment mechanism. This floating adjustment mechanism made the life-size shot practicable with the AF Micro Nikkor 55mm f/2.8 and then made it possible to take into consideration delicate imaging characteristics for the design in the AF Micro Nikkor 60mm f/2.8.

Now let's check what Ai Micro Nikkor 55mm f/2.8 can do. First, let's see the optical design sheets.

This lens features a very small spherical aberration in the shooting range from a long distance to a portrait distance. In addition, the astigmatism is also limited to a very small range, and the transverse chromatic aberration is successfully compensated as well. This provides aberration balance, showing a potential for high resolution and high contrast. Furthermore, the distortion is as well suppressed as would be expected for the micro-lens. The coma exhibits a distinctive characteristic to provide a little harder out-of-focus areas. Next, let's examine the aberration characteristics at the magnification from 1/10 to 1/2. We find that the spherical aberration increases a little and the coma turns into a sagittal coma. However, satisfactory sharpness is retained, and the image formation capability is appropriate. Rather, due to the coma effects, the harder defocus characteristics may be relieved to provide imaging characteristics suited for close-ups.

Here, let's examine the imaging characteristics of this lens based on the actual shots. First, let's discuss the end result picture taken at a long distance. At the maximum apertures of f/2.8 to f/4, some compressed contrast is observed due to the trace amount of flare, though moderate resolution is yielded in the images. At the apertures from f/5.6 to f/11, the flare disappears and the images show high contrast and high resolution. This aperture range is expected to provide such a crisp feel of sharpness characteristic of the Micro NIKKOR. At the apertures from f/16 to f/32, the images are susceptible to light diffraction and to gradually lower resolution. Next, let's discuss the short-range and micro-range photos. At the maximum apertures of f/2.8 to f/4, the flare is not so significant as in the long-range shots, and the images show higher resolution and sharpness. At the other apertures, similar imaging characteristics are obtained as in the long-range photos.

Next, let's check the photographic characteristics with sample pictures. Samples 1 and 2 were both taken with the maximum (fully open) aperture (f/2.8). Sample 1 (same as Sample 2 in Tale 25) shows a portrait. It was taken by getting slightly closer to the subject due to the micro-lens. As you can see from the reflection of the light in the pupil of the eye and the hair, it is clear that the lens offers very high resolving power and sharp contrast. The image provides a relatively hard gradation, offering a documentary style finish. This suggests that this lens allows the specific photographic representation by taking full advantage of the imaging characteristics.

Sample 2 (same as Sample 1 in Tale 25) shows the image in the shooting range expected to be the best for Micro NIKKOR. The subject in focus looks very sharp with crisp and clear reproduction of the metallic luster, showing a wide-range tone reproduction. In addition, we find that the lens provides satisfactory blurring and allows the most natural imaging in this shooting range.

This was an all-round lens allowing taking photographs at infinity to a magnification of 1/2. Many customers had reportedly purchased this lens as a daily use lens replacing the standard lens. This was attributed to the imaging characteristics mentioned above, especially the sharpness available with this lens. Mr. Hamanishi won a victory. He surpassed his teacher. I can imagine how glad he must have been.

Profile of Mr. Yoshinori Hamanishi

Mr. Hamanishi once had an uncommon (enviable?) career as a teacher at a girls' high school before joining Nikon. And, to tell the truth, Mr. Hamanishi was a good-looking man when he was young. This caused me to wonder why he gave up his career as a teacher at a girls' high school. I asked Mr. Hamanishi about it. However, he did not give me a definite answer to my question. He spoke guardedly and the question went unanswered. At last, however, Mr. Hamanishi began to talk quietly: "I became a teacher, full of ambition, only to find there were few students who were really interested in the physical sciences...."