NIKKOR - The Thousand and One Nights No.75
An exciting new photo system that changed the times!
IX-Nikkor 20-60mm f/3.5-5.6
Tale 75 features a lens for APS cameras that were the flowers of photo systems that never bore fruit. What is APS? How was it developed? APS had many hidden features that hinted at the future of digitalization. In this tale, we will look at the IX-Nikkor 20-60mm f/3.5-5.6 lens specialized for APS cameras.
What innovations did the IX-Nikkor introduce? And the end of APS. Let's refer to the IX-Nikkor to track the history of APS.
by Haruo Sato
I. The Advanced Photo System (APS)
The Eastman Kodak company invited four other camera manufacturers-Fujifilm, Nikon, Canon, and Minolta-to join them in the development of a new photo system that would come to be known as APS and represent a new global standard. The first APS-format film was released in April of 1996. Though just five companies were involved in the format's inauguration, several others soon joined in until it was temporarily as popular as the conventional 35mm format.
The primary feature of APS systems was the film. It utilized a dedicated film format known as IX240 that set a new standard. The "IX" in IX240 was short for "information exchange". A major distinction and selling point of APS systems was the film's ability to record information other than the image. This is comparable to the ability of modern digital cameras to record Exif headers with images. Information such as shooting settings, shooting date and time, print size and number of prints, and comments could be recorded on a magnetic surface that coated the film and could be used when prints were ordered. The "240" in IX240 referred to the film's width of 24mm.
Another important feature of the film was that the frame size (exposure area) measured 16.7 x 30.2 mm with a longer aspect ratio (16:9) than other conventional film formats. The basic image format was known as "H" for "high definition". All images were initially exposed at this 16:9 size. Users could also choose from two additional formats. The "C" and "P" image formats were formed by cropping the "high definition" image. The "C" format cropped images horizontally to achieve the classic 2:3 aspect ratio of conventional 35mm film. The "P" format cropped images vertically for panorama-type images with a 1:3 aspect ratio. The magnetic recording used by the system also supported automatic printing of each image at different sizes by simply specifying the desired print size with shooting.
The film cartridge also had its own unique characteristics. It is clearly smaller than 35mm film cartridges, but because it is a sealed cartridge, it can be loaded without touching the film and there is no light leakage. This means that the film can be exchanged as many times as the user likes during shooting. Even partially exposed film can be replaced, and shooting to the unexposed portions of the film can be resumed later. In addition, as fully exposed film cannot be loaded into cameras, users did not have to worry about accidents such as double exposures. This system offered a number of advantages, including its smaller size, possibilities for automation, and its affinity with minilabs. Many manufacturers released APS-format cameras at the same time in 1996, but those that best represented the concepts behind the system were APS compact cameras and disposable cameras. Nikon, Canon, and Minolta even began developing SLR cameras for the APS system. Nikon's first APS-format SLR was the Pronea 600i. It supported IX Nikkor interchangeable lenses exclusively with a dedicated mount that was based on the F mount. The next camera to be developed was the Pronea S, a camera with a completely new design equipped with a zoom lens known as a pancake zoom. However, 2002 saw the beginning of the end of the APS format, with major camera manufacturers starting to abandon the format. Kodak finally announced the end of its APS film production and sales in 2011. Why was the APS format abandoned? The reason for this was the tremendous progress made with digital cameras. The APS format had served its purpose. Everyone knows, though, that the DNA of the APS format lives on in digital cameras. Many are familiar with, and even own, digital camera systems based on a sensor size commonly referred to as the APS-C image sensor format. The concept of the Exif header also remains. Some consider the APS format a flower that never bore fruit. However, I believe that it played a major role in the transition to digital. The system did in fact disappear, but I think that it was indeed fruitful.
II. IX-Nikkor lenses
How did IX-Nikkor lenses differ from conventional F-mount Nikkor lenses? First of all, they looked different. One major difference was the area around the mount. Basically, a system with which the aperture is controlled by the camera was adopted. Therefore, just as with G-type AF lenses, IX-Nikkor lenses had no aperture ring. The flange focal distance was the same as that of the F mount. However, the fundamental difference was the dedicated IX-Nikkor lenses' backfocus. APS-format (H) film measured 16.7 x 30.2 mm. The diagonal of the image circle measured approximately 34.5 mm. That makes it more than 8.7-mm smaller than the 43.3-mm diagonal of the 35mm format image circle. This allowed for a smaller quick-return mirror, and meant that the lens' backfocus could be shortened accordingly. In order to make IX-Nikkor lenses smaller, the lens elements and base were packed into the rear portion of the lens barrel that was closer to the film plane than the lens mount. This caused the rear portion to protrude beyond the lens mount. As a result, while F-mount NIKKOR lenses can be used with Pronea cameras, it is mechanically difficult to mount IX-Nikkor lenses on F- and D-series cameras. Unfortunately, until quite recently, IX-Nikkor lenses, which have the same dimensions as the F mount but are not F-mount lenses, could not be used with cameras other than Pronea-series cameras. With APS-format film no longer manufactured, there seemed to be no future for APS-format cameras and lenses. However, a ray of light has appeared. That's right! The mirrorless Z system has arrived. For this tale, I was able to use the IX-Nikkor lens with a certain mount adapter that I modified slightly. Needless to say, I did it at my own risk. The use I describe later is not manufacturer approved! It did allow me, however, to revive a "dead" IX-Nikkor lens.
III. The IX-Nikkor lineup
What sort of IX-Nikkor lineup was available? As you know, normal G-type AF lenses for the F mount could be used with Pronea cameras. As the format was smaller than the 35mm format, however, telephoto shooting was easy and stress free, just as when using a Nikon DX-format digital camera. The issue, however, was with wide-angle shooting. A major objective behind the APS system was to make it more compact. Wide-angle lenses had to be smaller. Therefore, just as with the DX format, wide-angle lenses were pivotal in our development of the lineup. The IX-Nikkor lineup began with the 24-70mm f/3.5-5.6 zoom kit lens, and went on to include a 20-60mm f/3.5-5.6 wide-angle zoom lens and a 60-180mm f/4-5.6 telephoto zoom lens, all three of which were released to coincide with the 1996 release of the Pronea 600i. A second round of lenses was released to coincide with the 1998 release of the Pronea S. Those were the 30-60mm f/4-5.6 pancake zoom lens, a new version of the 20-60mm f/3.5-5.6, and the 60-180mm f/4.5-5.6. That was the complete IX-Nikkor lineup. Of these six lenses, I designed the optics for all but the 60-180mm lens-that is, the four wide-angle to standard zoom lenses-all on my own. Reducing labor and development costs was an important theme at the time.
IV. Development history and the designer
Let's take a look at the development history. Optical design began, and was completed, in April of 1995. I remember that design was rushed over a period of just several days. I designed the optics. Trial production began as soon as designs were completed. Trial production ended in September of 1995, and the transition to mass-production trials began in the same month. Mass production began in 1996, and the lens was released in September 1996. The same optical design was used for the new IX-Nikkor 20-60mm f/3.5-5.6, however, for which the improved Nikon Integrated Coating was adopted and the lens barrel design was modified. This new version was released in 1998. This tale is primarily about the new IX-Nikkor 20-60mm f/3.5-5.6.
V. IX-Nikkor 20-60mm f/3.5-5.6 construction and features
First, take a look at the cross-section of the IX-NIkkor 20-60mm f/3.5-5.6 (Fig. 1). Please forgive me if the following is quite technical.
The basic structure of this lens is the standard concave/convex (negative/positive) two-group zoom structure often used for wide-angle zoom lenses. It is compact, inexpensive, and easy to manufacture. The front (concave) group is labeled "G1" and the rear (convex) group "G2" in Fig. 1. Zooming (magnification) is performed by changing the spacing between the front and rear groups. The zoom trajectory is indicated by the arrows.
One of the primary features of this structure is that the aperture diaphragm (the aperture that determines the f/-number) is located outside of, and just in front of, the rear group G2. This structure makes aberration correction a little difficult, but as the rear group with its high sensitivity to decentration is not divided by the aperture, it is well suited to reliable and stable mass production. The lens is also equipped with a flare stopper (FS) nearest the imaging side. This flare stopper moves independently during zooming to effectively remove flare due to upper coma that commonly occurs at mid-range to telephoto focal lengths. The first element in the front group is a hybrid aspherical lens. This lens element was achieved not only through manufacturing means, but also with the development of a special resin that forms an aspherical layer of significantly varying thicknesses over a deeply concave surface. The key to this lens is the strong refractive power arrangement of each group. Both the front and rear group have a stronger refractive power than conventional wisdom would indicate. Therefore, the strong negative refractive power and aspherical shape of the first lens element are key characteristics of the design of this lens. The rear group is constructed of two convex elements and a doublet with a convex lens element at the front to greatly reduce spherical aberration and coma. By putting the concave surfaces of the two doublets together, the effect of a triplet with a concave lens at the center is achieved. As the two doublets are arranged as if the triplet's concave lens were divided into two, the sensitivity to multiple aberrations, such as spherical aberration and astigmatism, and the sensitivity to decentration are both reduced. Therefore, the lens has a high tolerance to lens aberrations and can provide even better compensation for curvature of field and astigmatism. This lens was applied for a patent in Japan in 1995.
As a result of these ingenuities, we were able to develop a compact and inexpensive zoom lens that covers the ultra wide-angle range and offers "3x magnification with the "two-group zoom" not available at the time. Nikon later released the 28-100mm (3.57x) two-group zoom, but at the time the IX-Nikkor 20-60mm f/3.5-5.6 was released, the maximum magnification possible from a two-group zoom structure was 2.8x, and most commonly just 2x. 3x and higher was unheard of. It was thought that even if such a lens could be designed, one would have to sacrifice either performance or production ease to realize it. Those were the conditions and misconceptions under which the compact standard zoom lens IX-Nikkor 20-60mm f/3.5-5.6 that covers a wide-angle to mid-telephoto range of focal lengths equivalent to 25 mm (with an angle of view measuring 2ω=82°) to 75 mm, and was easy to manufacture with a fewer number of lens elements, was completed.
VI. Design performance and evaluation
Now let's refer to design data to guess at rendering characteristics based on aberration characteristics. We'll begin at the maximum wide-angle position.
The most noteworthy characteristics are how little astigmatism there is, and how flat the curvature of field is, throughout the entire frame at the maximum wide-angle position. Curvature of field is kept to a slightly negative degree. In addition, there is little lower coma from the center of the frame to the edges, and upper coma is slightly over-corrected in the positive direction at the edges. The lens tends to exhibit the so-called outer coma. We tried to provide the same degree of compensation for these coma, astigmatism, and curvature-of-field tendencies throughout the full range of focal lengths. Concepts behind the design of this lens were to achieve good flatness, good point-image reproduction, and good frame uniformity. Both on- and off-axis chromatic aberration is well compensated without any negative consequences. Negative barrel distortion occurs to a degree of up to about -4% (APS-C format). Spherical aberration bulges slightly due to under correction. However, the amount of aberration generated by that bulge is not a problem at f/3.5.
Next let's take a look at performance at mid-range focal lengths around 35mm. Although the design concepts noted above are still maintained, curvature of field increases slightly. Specifically, the meridional image plane leans in the positive direction at the outermost edges due to the adverse effects of the aperture diaphragm being far from the concave lens group of the second group. This causes an increase in the "bulge" that results in positive bending of the image at high image heights. The full correction (aptitude correction) with which much spherical aberration remains at around the 70 percent position of the diameter was applied, but there is no problem with the diameter of the entrance pupil of f/3.5. Coma is good with little lower coma, but upper coma still tends to remain slightly at the edges of the frame. Distortion is almost nonexistent at -0.2%.
Finally, let's look at performance at the maximum telephoto position. We see good correction at the telephoto position with little astigmatism and curvature of field measured in slightly negative values throughout about 70% of the frame. This tendency is similar to the curvature-of-field correction tendencies seen at mid-range focal lengths. Spherical aberration is fully corrected with a sufficiently small amount generated considering the slow f/5.6 aperture. As for coma, a tendency toward inner coma changes at the maximum telephoto position. Lower coma becomes slightly under-corrected, and there is very little upper coma. Sagittal coma was basically non-existent at other focal lengths, but it is a little more noticeable at the telephoto position. Distortion measures around +0.4%. While this degree of distortion is barely noticeable, it has changed from negative barrel type to positive pincushion type. As for chromatic aberration, only axial chromatic aberration tends to become slightly under-corrected. There are no problems with focal points, but some color bleed may be attached to bokeh.
Further, while it occurs at all focal lengths, focusing on objects at short distances is performed by moving the front group (G1) toward the object. This causes aberrations to change in a similar way at all focal lengths. Short-distance fluctuations change both spherical aberration and curvature of field in the positive direction. Only distortion changes to negative distortion.
Now let's look at MTF performance. First we'll look at how contrast is reproduced at 15 and 40 lines/mm, which take photo enlargement into consideration. Observe MTF (15, 40 lines/mm) with focus at infinity at the wide-angle 20 mm position and maximum aperture. When focus is acquired at the center of the frame, excellent contrast is achieved in more than 78% of the frame (from the center outward). However, at 40 lines/mm, contrast drops slightly in the intermediate portions of the frame, and is maintained at around 40-60% to the extreme edges. A high value of 60-70% is maintained at 15 lines/mm, so it seems that the lens has no problem reproducing contrast. Contrast remains stable with no sudden drops, even to the extreme outer edges of the frame. It could be said that this characteristic is the result of striving for the ability to reproduce uniform image quality and contrast, with no unevenness, from the center of the frame to its outermost edges. The lens successfully produces a flat plane, reproduces point images, and achieves consistent image quality throughout the frame, which we considered the most important aspects of optical performance, especially for wide-angle lenses.
Next let's take a look at contrast reproduction under similar conditions at the same 15 and 40 lines/mm at the mid-range 35 mm focal length. At 40 lines/mm, contrast measuring approximately 63% is maintained throughout most of the frame with the exception of a slight drop at the center. Contrast at 15 lines/mm increases to more than 82%, likely making issues like flare less of a problem. The effects of curvature of field and outer coma tendencies cause contrast to drop from the intermediate portions of the frame to the edges, but at 40 lines/mm, contrast reproducibility holds at 30-50%. At 15 lines/mm, a higher value of 70% is maintained to the extreme corners of the frame. The most noteworthy characteristic of performance in mid-range focal lengths is not extremely high contrast, but rather contrast of a sufficient level that does not drop sharply and remains consistent to the edge of the image circle. The lens can be described, in a good way, as one that maintains a flat plane.
Finally, let's look at contrast reproduction under similar conditions at the same 15 and 40 lines/mm at the maximum telephoto position of 60 mm. When focus is acquired at the center of the frame, excellent contrast is achieved in more than 74% of the frame (from the center outward). Then, a tendency toward inner coma appears and contrast gradually decreases. Contrast drops to 40-60% at 40 lines/mm in the intermediate portions of the frame, and to about 30-50% at the edges. However, at 40 lines/mm, contrast measuring 20-40% is maintained even in the outermost corners with no sudden drop. What's more, contrast at 15 lines/mm gradually drops from 80% to around 50% from the intermediate portions to the extreme edges of the frame. This means that images captured at the maximum telephoto position tend to appear somewhat soft with a little flare. This tendency is actually fine for portraits and photographing products or other objects. Basically, wide-angle to standard zoom lenses are designed with the assumption that the wide-angle positions will be used for landscape photography and the telephoto positions for portrait photography. In that respect, I think that this lens was produced with the desired aberration balance.
VII. Actual performance and sample images
Next let's look at results achieved with some actual images of distant scenes. I used the Nikon Z 7 with a mount adapter that I modified slightly to enable mounting of the IX-Nikkor 20-60mm f/3.5-5.6. The modifications I performed were completely experimental, and I checked operation carefully. I did it at my own risk. Such operation is not recommended, or even approved, by Nikon.
The IX-Nikkor lens covers an area of the frame measuring 16.7 x 30.2 mm. For convenience, I captured these sample images using the DX-format image area. Please understand that the angle of view is slightly more narrow.
I will detail performance at each aperture setting separately. Evaluations are subjective, and based on individual preferences. Please keep in mind that my opinions on sample images and evaluations are for reference purposes only.
Maximum wide-angle 20 mm position
f/3.5 maximum aperture
Generally speaking, the lens offers adequate resolution and contrast from the center of the frame to the edges. Images are also sufficiently sharp. If you look closely, you can see that a little flare that increases gradually is generated toward the edges of the frame. In addition, some cyan-colored flare is generated in backlit portions. However, sufficient resolution is maintained to the extreme edges of the frame.
f/4 to f/5.6
By simply stopping down the aperture one stop, flare at the edges of the frame disappears and contrast increases. Image quality, especially in terms of contrast and resolution, increases not only at the center of the frame, but also at the edges. Colored flare in backlit portions is also less noticeable.
f/8 to f/11
The image seems to exhibit even better image quality. These aperture values provide the best image quality at the wide-angle position for this lens.
f/16 to f/22
Image quality clearly suffers due to the effects of diffraction.
Mid-range 35 mm position
f/4.4 maximum aperture
Image quality is as if the good portions at the center of the frame at the maximum wide-angle position were cropped out and enlarged. What is special about it is the small amount of flare evenly distributed from the center of the frame to the edges. Images don't exhibit harsh contrast with landscapes and the like, but rather a more pleasing degree of contrast. Resolution is good and nearly consistent throughout the entire frame.
f/5.6 to f/8
The small amount of flare that was present throughout the frame disappears for images that are sharper and clearer. However, contrast is not too harsh. Images look a little sharper.
f/11 to f/16
Uniform performance throughout the frame is good. However, image quality tends to suffer at f/16.
f/22 to f/32
Image quality clearly suffers due to the effects of diffraction.
Maximum telephoto 60 mm position
f/5.6 maximum aperture
Resolution and contrast at the center of the frame are good. Flare occurs, and increases gradually, closer to the edges of the frame. Both contrast and resolution tend to drop. However, I like the fact that there is no colored flare and no great reduction in image quality.
f/8 to f/11
By simply stopping down the aperture one stop, flare at the edges of the frame disappears and contrast increases. Flare seems to have disappeared completely throughout the frame and contrast looks more consistent. At the maximum telephoto position, the best image quality is achieved at f/11.
f/16 to f/22
Performance deteriorates due to the significant effects of diffraction. I cannot recommend using such small apertures.
Now let's confirm these rendering characteristics with some sample photos.
These images have not been edited or enhanced so that you may judge the characteristics of this lens for yourself. For the most part, the images are portraits and landscape snapshots because I assume those to be the most common types of photos for which this lens would be used. I included backgrounds in the images again, and adjusted shooting distances slightly, to better show three-dimensional rendering characteristics.
Sample 1 was captured at the wide-angle 20 mm focal length with the aperture set to the maximum aperture of f/3.5. I chose clothing and a background that would clearly show the lens' resolving performance. If we look at the foliage and trees, the model's face and hair, and the pattern on her jacket, we see that the image exhibits a sufficient degree of sharpness. There is also no clear drop in sharpness in the intermediate and peripheral portions of the frame. The overall impression is good with very little color bleed along the focal plane.
Sample 2 was captured at the mid-range 35 mm focal length with the aperture set to the maximum aperture of f/4.4. As with the image captured at the maximum wide-angle position, image quality is good with sufficient sharpness and detail, as well as a pleasing degree of contrast. It is clear that the lens handles the sharp portions of the image well without making them too harsh. I am satisfied with the sharpness, but the background has become a little unpleasant. Bokeh is a little hard.
Sample 3 was captured at the maximum telephoto 60 mm position with the aperture set to the maximum aperture of f/5.6. The degree of sharpness exhibited is more than sufficient. Even a close look at the pattern in the model's jacket proves a lack of color bleed. The high resolution of the lens is proven by the detail expressed in the model's hair. The only real disadvantage is the harshness of the bokeh. The tendency toward the "bubble bokeh", which is so popular these days, is consistent all the way to the edges of the frame. We may have put too much emphasis on increasing sharpness. I regret this now.
Sample 4 was captured at the wide-angle 20 mm position with the aperture set to the maximum aperture of f/3.5. I focused on the lights for this image. Viewed at 100%, it is clear that the lights are in extremely sharp focus. The buildings and signs in the foreground, on the other hand, are slightly out of focus. The image is clear with no noticeable color bleed, making the lens good enough for this type of scene under these conditions.
Sample 5 was also captured at the wide-angle 20 mm position with the aperture set to the maximum aperture of f/3.5. This time I focused on the wall. Focus seems to be slightly in front of the intended focal point. There is no peripheral illumination falloff or noticeable color bleed. Good contrast and clear rendering make the lens good enough for this type of subject and situation.
Sample 6 was captured at around the mid-range 28 mm focal length with the aperture set to the maximum aperture of f/4. I think focus was acquired on a position between the trees and the bench. Flare is reflected in the sunlight through the trees, but contrast is sufficient, and the image is quite clear.
Sample 7 was captured at around the mid-range 35 mm focal length with the aperture set to the maximum aperture of f/4.4. I think I focused on the bench. If you look closely, you can see flare in the sunlight through the trees here as well. However, you can also see that there is no color bleed, contrast is good, and the image is clearly rendering, allowing the lens to stand up to this type of subject and situation.
Sample 8 was captured at the telephoto 60 mm position with the aperture set to the maximum aperture of f/5.6. This time, I think I focused on the trees. If you look closely, you'll see that the image is very sharp. You can even read the facial expression of the man walking his dogs. You can see that contrast is good, the image is sharp and clear, and it exhibits sufficient resolution.
Sample 9 is a close-up that was captured at the maximum telephoto 60 mm position at maximum aperture. One of the advantages of this lens is how it easily enables macro photography. This image was taken at the minimum focusing distance (R = 0.35 m). As you can see, the lens is easily capable of adequate performance with close-up shooting. Even the bokeh characteristics are pleasing.
VIII. A man named Yasuhiro Aono
At one time, I was secretly asked to work with Kodak, the inventor of the APS system. After that, there were heated discussions within the company. As a result, a team led by Naoki Tomino was established to develop the APS system and get it on track. It was not a traditional development team that develops items in accordance with a schedule and plan. It was a small group of selected developers working together for the success of the APS system. I became a member of the group and was instructed to design the standard and wide-angle zoom IX-Nikkor lenses. I signed a number of non-disclosure agreements, learned more about the APS format, and set the standard for lens design. I was recommended for the position by my boss at the time, Yasuhiro Aono. Aono was the first person I asked to teach me about optics when I began working at Nippon Kogaku K. K. (now Nikon Corporation). The connection we established continued until his retirement. He was a manager of the optical design department, manager of the camera design department, and then served as a director for Tochigi Nikon Precision Co., Ltd. After returning to headquarters from Tochigi Nikon, he was honored with appointment as a Nikon Fellow for his outstanding achievements not only at Nikon, but in the camera and imaging industry as a whole, and for his contributions to academic societies. Aono was not actually involved in the field of optical design. He was initially involved in research. However, he was transferred to the optical development section of the newly inaugurated Optical Designing Department I mentioned in Tale 71. He had a wide range of knowledge including optical design and theory and developed a proprietary software application that became the most widely used in optical design throughout Nikon. When I joined the company, he was designing and developing optics for high-definition TV (HD TV lenses) in cooperation with a broadcasting station. I took his place. That was more than 25 years ago. I felt that I could discuss anything with Aono, so in retrospect, I might have annoyed him with my straightforward comments, but he kindly watched over me. I'll share with you one story about my mentor, Yasuhiro Aono.
I heard this story from someone else. Aono had the will and strength to see something he believed in through to the end. At one point, he was proposing a project based on a business plan to an executive. No matter how hard Aono tried to persuade the executive, he could not get the nod. However, he would not back down. In the end, the executive got angry and asked, "Do you not hear me?", and told him, "You're on your own!" Aono thanked him, said he would proceed on his own, and left. Receiving the response he did from an executive would cause most people to apologize and withdraw. In the first place, most people would never anger an executive for fear of losing their job. However, Aono was sure he was right. He never gave up. I just love this story. It makes me want to be just like him. Aono quit working when he retired, but he is still very healthy and active in his photographic pursuits. He didn't take many pictures when he was working with lenses, but his retirement lit a fire in him, and he now spends his days taking pictures. I was surprised at the level of his photographic skills. His images look like they were captured by a pro. I hope that he will continue in his efforts to cultivate photo culture.