NIKKOR - The Thousand and One Nights No.61

The AI Zoom-NIKKOR 50-135mm f/3.5S

Tale 61 introduces the AI Zoom-NIKKOR 50-135mm f/3.5S, the zoom lens that carried development of high-power zoom lenses before autofocus (AF) systems were introduced. These were the days of the Nikon F3. Key features of the new AI-S line of lenses were their higher-power zoom capabilities and their more compact size. This tale provides a look at the hidden secrets of the AI Zoom-NIKKOR 50-135mm f/3.5S, an AI-S lens with a unique design.

by Haruo Sato

I. History of AI Zoom-NIKKOR 50-135mm f/3.5S

First, let's take a look at the history of the AI Zoom-NIKKOR 50-135mm f/3.5S. The AI Zoom-NIKKOR 50-135mm f/3.5S was released in April of 1982. From the beginning, it was a so-called AI-S lens equipped with the new auto indexing (AI) system. This was also the age of the Nikon F3. It was shortly after this that the shift to AF occurred.
During these turbulent times, the AI Zoom-NIKKOR 50-135mm f/3.5S was left by the wayside, never experiencing rebirth as an AF lens. Its successor was a 35-135mm f/3.5-4.5 lens. It evolved into a normal high-power zoom lens with a range of focal lengths that began at the wide-angle 35mm. Ultimately, the AI Zoom-NIKKOR 50-135mm f/3.5 was sold for roughly just two years, surviving for just one generation before it was replaced. This is a very short lifespan for a NIKKOR lens.

II. Development history and the designer

AI Zoom-NIKKOR 50-135mm f/3.5S optics were designed by my mentor, Mr. Kiyoshi Hayashi, previously introduced in Tale 31. If Mr. Hayashi, Mr. Tsunashima, and Mr. Mori were the primary designers behind the NIKKOR Auto line of lenses, Mr. Hayashi was one of the primary designers behind Nikon's new AI NIKKOR lenses. Though Japan's great lens designers are not generally known, we can trace their work through reports, development histories, and patents. Now let's take a look at the history of development. The optical design report for this lens was submitted in 1981. Trial production drawings were issued in December of 1980. Mr. Hayashi recalls starting work on lens optical design in the spring of 1980. The transition to mass production was made in the early summer of 1982, and the lens was finally released in November 1982.

III. Lens construction and characteristics

First, take a look at the cross-section of the AI NIKKOR 50-135mm f/3.5S (Figure 1). Please forgive me if the following is quite technical. This is a four-group (positive/negative/negative/positive or convex/concave/concave/convex) zoom lens. Many readers may be unfamiliar with this construction. This zoom method (type) is known as the "yamaji" type. From left in Figure 1, elements four through six form the variator concave lens group. Immediately following that is a concave/convex doublet that is the compensator for the concave group that is the primary feature of this lens. First of all, it should be noted that the time-tested standard for telephoto zoom lenses was the positive/negative/positive/positive (convex/concave/convex/convex) afocal-type lens that was a major invention of Mr. Nakamura (see Tale 42). For Nikon as well as our rivals, in Japan and abroad, this lens took the world by storm. Even today, this afocal zoom type serves as the de facto standard for fast telephoto zoom lenses. The "yamaji" type challenges the afocal zoom type, which, in theory, tends to become quite large and is not suited to wider angles of view. I'll use the following illustrations to explain the primary characteristics of the "yamaji" type. Take a look at Figures 2 and 3.

Figure 2 is a diagram of a positive/negative/negative/positive (convex/concave/concave/convex) "yamaji" type lens. Figure 3 is a diagram of Mr. Nakamura's positive/negative/positive/positive (convex/concave/convex/convex) afocal-type lens. Both are constructed with, from left, a focus group, a variator group, a compensator group, and a master lens group. The variator group moves a great deal to change magnification, and the compensator group compensates for focus movement. The first thing we see with these structures is that they must be quite large to support a high zoom ratio because the distance lenses must move increases as the zoom ratio increases. I would like you to note the path that the compensator group takes. With the afocal zoom type, the compensator group moves with a path that is convex in relation to the focal plane (right side of the diagram). With the "yamaji" type, compensator movement is concave in relation to the focal plane. Therefore, the space indicated as "d3" in the diagrams can be made smaller with the "yamaji" type (Figure 2) than with the afocal zoom type (Figure 3). This makes a smaller lens possible. However, this reasoning is accompanied by one major trap. Because the "yamaji" type is constructed with adjoining variator group and compensator group that are both concave, light used for imaging must be bent significantly by placing the power of a large convex group immediately after them. Therefore, a structure with which the master lens group was convex in nature was required. Even though the total length of the lens could be shortened, it was not certain that the "yamaji" type would always result in a smaller lens. Nonetheless, the characteristic that offers the most in terms of possibilities is that both the variator and compensator groups have the concave nature. This means that the concave power of the part that changes magnification can be increased. This characteristic also contributes to optimization of the Petzval sum. Some of you may have already guessed at another advantage of the "yamaji" type. That's right! This lens type is well suited to wider angles of view. This was the secret behind our ability to reduce the maximum wide-angle focal length to 50mm with the AI Zoom-NIKKOR 50-135mm f/3.5S.
However, even wider angles of view require a negative (concave)-lead zoom structure or a zoom lens structure with which the first convex group moves. I think the reason this lens type was not used much with 35mm format zoom lenses was due to the invention of the positive (convex)-lead type of zoom lens with which the first group moves. The "yamaji" type did take off in the world of television cameras that require much higher zoom power and a fixed total length. These days, the majority of high-power zoom lenses for video cameras (high-end professional video cameras) are constructed with this "yamaji" type. It has become the standard lens type for video camera lenses.

So, why did Mr. Hayashi use this lens type in the early days of high-power zoom lenses? When I asked him before, he said he did so because he wanted to satisfy technical interests and to examine the possibilities and limits of the "yamaji" type. His adoption of the "yamaji" type resulted in a high-power zoom lens that offered good optical performance and was not too big. However, this was the only lens for which the "yamaji" type was adopted. Mr. Hayashi said the structure of the master group had to be quite complicated, and was unable to make lenses for 35mm cameras as small as he had hoped. In addition, there were difficulties achieving wider angles of view. Through this design, Mr. Hayashi identified the limits of this lens type. It is interesting to see a coincidence. Mr. Hayashi's curiosity led to a zoom lens that was extremely rare and unique in the NIKKOR lineup.
Even today, it likely falls in the category of rare products.

Now let's take a look at the aberration characteristics of the AI Zoom-NIKKOR 50-135mm f/3.5S. We'll begin at the wide-angle 50mm position.

The first thing we notice is that the lens is designed to reduce astigmatism in the portrait range. This sort of focusing system, with which the first group moves back and forth, exhibits some close-range aberration fluctuation. In short, it provided an opportunity for the designer to show their skill by how well remaining aberration is preserved. My mentor, Mr. Hayashi was a true craftsman. He achieved a good aberrational balance that considered the shooting time, purpose, and occasion and kept any reduction in sharpness to a minimum.
His design clearly gave away the interest in photography he'd had from his youth. Spherical aberration at the wide-angle position is near-perfectly corrected, and there is little close-range aberration fluctuation. It is exemplary and worthy of a sample image that might be found in an optical textbook. Coma is well compensated. Color shift caused by coma is skillfully suppressed, and I can find no fault there. Distortion measures approximately -3.9%. While this is a little high, it has a gentle barrel shape that is easily resolved with image processing. Now let's take a look at performance around the mid-range 90mm focal length. Spherical aberration is a little overcompensated at infinity which resembles to Wakimoto balance, the balance designed by Mr. Wakimoto. It is displaced in the negative direction with shooting at close ranges. With landscapes, stopping down the aperture a little results in sharper images, and portraits have pleasing blur characteristics (bokeh) for an aberration balance that follows Mr. Wakimoto's model. I should also mention that lateral chromatic aberration, coma, and color shift caused by coma are all skillfully compensated, making the AI Zoom-NIKKOR 50-135mm f/3.5S a lens that could be used even in today's digital age. Finally, let's look at results at the 135mm maximum telephoto position. There is hardly any spherical aberration with shooting at infinity, soaring like super-telephoto spherical aberration charts. That is gradually displaced in the negative direction as the shooting distance decreases. Coma is nearly completely compensated, but some inward coma does remain. The aberration balance is very similar to that of a super-telephoto lens in terms of spherical aberration, chromatic aberration, and coma. The form aberration takes with shooting at infinity is exactly the same as with the AI NIKKOR 300mm f/2.8 internal focusing (IF) super-telephoto lens developed by Mr. Hayashi and others. I think that this distinctive way of designing lenses became a part of Mr. Hayashi. Only the difference is the aberration compensation tendencies in close-range. Aberration, especially spherical aberration, is displaced in the negative direction because the convex group moves forward with focusing. This tendency is actually fine for portraits and photographing products or other objects.

Now let's look at point formation. Surprisingly, this lens exhibits the same point formation tendencies at all focal lengths. To tell the truth, I was astonished that Mr. Hayashi had considered performance so thoroughly. The lens' ability to achieve the same quality of point formation at all focal lengths means that there is no sense of incongruity caused by differences in image quality with zooming. That is a superior design concept. Sagittal coma is suppressed so that points are formed with a triangular shape. However, there is no flare. While the center is surrounded by some flare, there is a core for a shape close to ideal. This lens reproduces points as points. Was this a miracle or did Mr. Hayashi make it to be so? The points all have exactly the same shape. I feel like Mr. Hayashi has taught me something new through this lens.

Now for MTF values. Contrast is good for an f/3.5 zoom lens at both 10 and 30 lines/mm. Especially noteworthy is the fact that the highest 10 lines/mm MTF values are achieved throughout the entire frame and at all image heights. This is also clear from flare-free point-formation characteristics. It may be that this was Mr. Hayashi's goal. In addition, 30 lines/mm MTF is consistent all the way to the edges of the frame in the portrait range. At the telephoto position, subjects captured at close ranges are softly rendered, and blur characteristics tend to improve at close ranges.

IV. Actual performance and sample images

Next let's look at results achieved with actual images. Details regarding performance at various aperture settings are noted. Evaluations are subjective, and based on individual preferences. Please keep in mind that my opinions are for reference purposes when viewing sample images and reading the evaluations.

Maximum wide-angle 50mm position

f/3.5 maximum aperture

There is flare at the center, but a resolution is relatively consistent from the center of the frame to the edges.
Resolution is not conspicuously high, but it does achieve fairly decent image quality. There is little color bleed.

f/4 to f/5.6

Stopping down the aperture to f/4 eliminates the flare at the center and increases contrast. At f/5.6, sharpness is increased to the edges of the frame.

f/8 to f/11

Consistent rendering is achieved throughout the entire frame. As contrast is greatly increased, images exhibit a little bit too much contrast. Of all aperture settings, the best image quality is achieved at f/8 to f/11. An aperture setting of f/11 is best for landscape photos.

f/16 to f/32

Even more consistent rendering is achieved throughout the entire frame, but resolution decreases. At f/22 to f/32 especially, the effects of diffraction are visible and resolution drops slightly.

Mid-range 85mm position

f/3.5 maximum aperture

There is a little more flare that results in softer rendering than is achieved at 50mm. Relatively consistent resolution is exhibited from the center of the frame to the edges. Resolution is not conspicuously high, but it does achieve fairly decent image quality. Even at this focal length, there is little color bleed.

f/4 to f/5.6

Stopping down the aperture to f/4 reduced flare and increases contrast. Flare is nearly completely eliminated at f/5.6, and sharpness is increased to the edges of the frame.

f/8 to f/11

Consistent rendering is achieved throughout the entire frame. Contrast is greatly increased, but a softness remains. Of all aperture settings, the best image quality is achieved at f/11. An aperture setting of f/11 is best for landscape photos.

f/16 to f/32

Even more consistent rendering is achieved throughout the entire frame, but resolution decreases. At f/22 to f/32 especially, the effects of diffraction are visible and resolution drops slightly.

Maximum telephoto 135mm position

f/3.5 maximum aperture

There is a lot of flare that results in soft rendering. A little sharpness and detail is lost. However, rendering is still decent for pleasing results. Even at this focal length, there is little color bleed.

f/4 to f/5.6

Resolution is increased by stopping down the aperture to f/4. Flare is nearly completely eliminated at f/5.6, and sharpness is increased to the edges of the frame.

f/8 to f/11

Images seem to suddenly become sharp at f/8. Consistent rendering is achieved throughout the entire frame.
Contrast is greatly increased, but a softness remains. Of all aperture settings, the best image quality is achieved at f/11. An aperture setting of f/11 is best for landscape photos.

f/16 to f/32

Even more consistent rendering is achieved throughout the entire frame, but resolution decreases. At f/22 to f/32 especially, the effects of diffraction are visible and resolution drops slightly.

If sharpness is the goal, the best results would likely be achieved at an aperture setting of f/8 to f/11 at all focal lengths (positions). For portraits, I think that I would use a setting between f/3.5 and f/4.

Now let's confirm these rendering characteristics with some sample photos.

So that you may judge the characteristics of this lens for yourself, compensation for lateral chromatic aberration, axial chromatic aberration, and vignetting has not been applied, and image sharpness has not been enhanced.

Sample 1 was captured at the wide-angle 50mm position at the maximum aperture of f/3.5. The girl's face and hair exhibit sufficient sharpness to make this a pleasing image. The leaves at her feet are also sharply rendered. Blur is not very soft, but it is not degraded quite to the point of double-line blur. The balance between light and dark (contrast) is also good for rendering that leaves little or no room for complaint.

Sample 2 was captured at around 60mm with an aperture setting of f/5.6. The image exhibits sufficient sharpness, clearly reproducing the girl's hair and even the fibers of her coat. The gentle blur is also pleasing.

Sample 3 was captured at around 85mm at the maximum aperture of f/3.5. This lens exhibits little difference in rendering, and a stable degree of sharpness with zooming. Blur characteristics are a little harsh, and while we do see a tendency toward double-line blur, I think the results we see indicate that this was an excellent zoom lens for its time.

Sample 4 was captured at the maximum telephoto 135mm position at the maximum aperture of f/3.5. It is a pleasing image that exhibits the necessary degree of sharpness in portions that are in-focus-namely the girl's eyes and hair. At telephoto positions, blur characteristics are softened, and tendencies toward double-line blur are greatly reduced.

V. Mr. Hayashi's character

If Mr. Higashi, Mr. Murakami, and Mr. Wakimoto were the founders of NIKKOR lenses for S-age cameras, Mr. Shimizu, Mr. Nakamura, Mr. Mori, and Mr. Tsunashima were the fathers of the NIKKOR Auto line. Further, Mr. Hayashi, who had a major role in this tale, Mr. Hamanishi, Mr. Fujie, and Mr. Wakamiya were the champions of the new NIKKOR lenses. I was lucky enough to have learned from Mr. Wakimoto, and to have been led by all of the NIKKOR masters, including Mr. Shimizu and Mr. Nakamura. Mr. Hayashi worked especially hard to teach me all that I needed to know, despite my rough start. That is why I came to call him my mentor. Working with and learning from such masters as Mr. Hayashi has allowed me to work in the field of optical design for more than thirty years. Further, the tales I write for NIKKOR The Thousand and One Nights are based on the knowledge I have cultivated working with such people. I still spend time with Mr. Hayashi, even though he is now retired. I even discussed this Tale 61 with him, and got a little behind-the-scenes information on AI Zoom-NIKKOR 50-135mm f/3.5S from him. Once a year, Mr. Oshita and I, together with some other colleagues, hold a meet-and-greet Mr. Hayashi. Mr. Hayashi's latest interest seems to be Baroque music. The other day he told me that he had acquired a rare Vivaldi opera. He was excited as a young schoolboy! His eyes still sparkle as they did twenty years ago. Music is his lifelong friend and companion. There is still much I would like to learn from Mr. Hayashi. I wish him a long and healthy life.