NIKKOR - The Thousand and One Nights No.65
The AF Zoom-Nikkor 28-200mm f/3.5-5.6G IF-ED
With Tale 65, we'll examine the AF Zoom-Nikkor 28-200mm f/3.5-5.6G IF-ED, the world's smallest and lightest high-power zoom lens that has experienced a quiet comeback of late. Not long ago, friends and acquaintances would often tell me that the ED 28-200 G was popular on the secondhand market, and that it was creating quite a buzz on the Internet. Many have asked me to tell them about this lens. The truth is, however, that the complete story of this lens cannot be told in a single Tale. This high-power zoom lens challenged all limits with thoughts and obsessions of developers. In this Tale, we will delve into the secrets of the AF Zoom-Nikkor 28-200mm f/3.5-5.6G IF-ED, a high-power zoom lens that is currently regaining its popularity.
by Haruo Sato
I. The first all-purpose high-power zoom lens
Supposing the 43-86mm was the first normal zoom lens, as I noted in Tale 63, just how was the transition from normal zoom to high-power zoom made? Zoom lenses grew and expanded in a variety of ways. While a negative-lead structure dominates wide-angle zoom lenses, a positive-lead structure with a fixed total length is most common for telephoto zoom lenses. Aspects of both wide-angle and telephoto zoom lenses have been adopted for normal zoom lenses, and a variety of structures have been invented in their development. They generally take one of two paths. The first is the speed path that enables a faster (brighter) lens, and the second is the high-power path for higher-power zoom. There were two major questions, however, when considering all-purpose lenses. The first was just how fast (bright) they can be, bearing in mind that the most standard lens has a focal length of 50mm and a maximum aperture of f/1.4. The second was whether a single lens could offer a broad focal length span, from the maximum wide-angle position to the maximum telephoto position? These questions, which naturally come to users' minds, resonated with the inquisitive spirit of designers. Following the high-power path results in progression from 35-70mm to 35-105mm, once to 28-85mm,and then from 35-135mm to 35-200mm. A positive-lead structure is advantageous when expanding the telephoto range. However, such structures make expansion in the wide-angle direction difficult. How can wide-angle focal lengths be expanded from 35mm to 28mm and 24mm? It would be possible to design lenses that supported these wider focal lengths if focusing at infinity were the only consideration. Unfortunately, the structure popular at the time—one with which focus was performed by extending group 1—would increase the size of the front element and yet the close-up performance and peripheral illumination would be sacrificed. All optical designers were battling this issue. Then, a ray of light appeared. That ray of light was the invention of the internal focusing (IF) system.
As many likely know, generally speaking, focusing must take place in front of the group that changes the variable power (i.e., closer to the photographic subject). Otherwise, significant gaps in focus will occur with focusing. These gaps in focus are allowed with varifocal lenses. That was when the ray of light appeared. Kiyotaka Inadome, who worked in the First Optical, Optical Department at the time, came up with the idea of an internal focusing cam system. The following is a simple explanation of the internal focusing cam system. With internal focusing, the amount of extension needed to focus at close distances varies by focal length. For example, at a shooting distance of 2 m, the focusing group must move 1 mm at the 28mm position and 8 mm at the 200mm position. To prevent loss of focus during zooming, this difference (the difference in the amount of movement required of the focusing group) should be compensated as the lens is zoomed in or out. This consideration led to the idea of incorporating an additional cam (a compensation cam) into the lens barrel. Mr. Inadome designed the Ai AF Zoom-Nikkor 35-70mm F2.8S that was the subject of Tale 39. He guided and helped me so much that I often thought of him as an older brother. It was in Mr. Inadome's day that we were able to extend the wide-angle position of high-power zoom lenses to 28mm and then 24mm, and the AF Zoom-Nikkor 28-200mm f/3.5-5.6G IF-ED was developed.
II. Development history and the designer
My much younger self designed the optics for the AF Zoom-Nikkor 28-200mm f/3.5-5.6G IF-ED. At the time, I often designed and developed wide-angle and normal zoom lenses. This lens was one that I really had to work hard on to get to mass production. It was a lens whose final release required the support of many co-workers and associates, and one that I remember well. I learned about cam design from Mr. Inadome, and at the trial production stage, I spent days locked in the lab with the legendary Mr. Udagawa considering ways to mass produce the lens. Mr. Udagawa and I also developed alignment equipment needed for mass production. That system became the foundation for later production systems used by Nikon.
We began designing the lens as soon as we returned to work after the New Year's holiday in 2001. The design was fortuitously completed on my birthday, February 21, 2001. Trial production began in May of 2001, with trial mass-production initiated in November of the same year. It was with trial mass-production that we encountered difficulties. Eventually, we achieved and maintained an adequate level of performance, and NTC, Nikon's factory in Thailand, successfully made the transition to full mass-production so that we were able to release the lens to customers in good conscience in September, 2003. Before that, I had to make several trips to Thailand to get the factory on track with mass production. Of all the lenses I've designed and developed, this one was by far the most difficult.
III. What makes a truly all-purpose high-power zoom lens?
Between 1996 and 2000, many manufacturers released high-power zoom lenses. All-in-one high-power zoom lenses were considered optimal for vacation and travel, and even everyday lenses. However, I had three complaints about these lenses. The first was image quality. This issue has resolved itself over time. The second was the fact that closest focusing distances were quite far. I thought they should support closer shooting, even if not as close as is possible with macro (Micro) lenses. Up to this point, it was difficult to photograph not only flowers and small animals, but even cuisine on a table. My biggest complaint, however, was the total length of the lens and the diameter of the barrel. In short, they were very large. It's only natural to want to take pictures that include meals and the dinner table when traveling. What's more, the cameras at that time were quite small, and many had a built-in flash (pop-up flash). When a long, large-diameter high-power zoom lens was used, it often shaded light from the built-in flash, and this shadow of the lens was reflected in photos. This was unacceptable, truly unacceptable. This, of course, was the age of silver halide film. Normal film has a sensitivity of ISO 400–800 at the highest. High-power zoom lenses are relatively slow (dark). Therefore, it was very difficult to use a high-power zoom lens without a flash. This made it necessary for photographers to carry a large external flash with them. My determination that there was no truly all-purpose high-power zoom lens was based on these facts.
A request for a high-power zoom lens design fell on my desk at a time when I was constantly considering this dilemma. Here was my chance! I challenged myself to design a lens that was not only smaller and lighter than any high-power zoom lens to come before, but also offered the shortest closest focusing distance ever for such a lens. I set my targets. I decided on a maximum length at the maximum wide-angle position based on the angle of coverage of the flash built into Nikon's smallest cameras at the time, the Nikon F65 and F55, and a closest focus distance of 0.45 m, the shortest at that time. These targets were considered impossible, but the completed lens actually ended up with a closest focusing distance of 0.44 m throughout the entire zoom range (supported focusing all the way to the macro range with a reproduction ratio of 1:3.2 at the telephoto end) The total length was also reduced to just 71 mm.
However, the drawback to these achievements was decreased productivity. At that time, very few optical systems utilized four aspherical surfaces on three lens elements. The cost of manufacturing aspherical elements was not so much the problem as was the precise adjustment required with assembly. In fact, we had not yet reached the level of precision adjustment this lens required. Therefore, we had to establish a new adjustment and alignment system, and then develop the necessary equipment. It was during trial production that Mr. Udagawa made his appearance, which felt a little like the second coming. Mr. Udagawa and I came up with the new adjustment and alignment system, which we modified time after time with testing and verification. With Mr. Udagawa's help, I was able to develop the adjustment and alignment system, and the equipment platform, upon which today's system is based.
IV. Lens construction and characteristics
First, take a look at the cross-section of the AF Zoom-Nikkor 28-200mm f/3.5-5.6G IF-ED (Fig. 1). Please forgive me if the following is quite technical. This lens utilizes a four-group, positive-negative-positive-positive (convex-concave-convex-convex) structure with which all groups move. This structure is best suited to making wide-angle lenses smaller. The structure and type are the secrets behind the AF Zoom-Nikkor 28-200mm f/3.5-5.6G IF-ED.
Optical designers generally think that in order to make a lens smaller, the refracting power of each lens group must be increased and a high rate of magnification should be used. This way of thinking is basically correct. However, it can be a trap. Increasing the refracting power of each lens naturally leads to an increase in aberration. Designers normally increase the number of lens elements used to compensate for aberration that does occur. As a result, the optics would utilize more lens elements that increased the weight of the lens. Ultimately, the lens would not be as small as the designer had hoped. It can be somewhat of a vicious cycle. The lens could not be made as small as possible using conventional thinking, regardless of the labor and cost put into it.
So, how would we make the AF Zoom-Nikkor 28-200mm f/3.5-5.6G IF-ED smaller? This lens was designed using a way of thinking that completely contradicted designers' common ideas and design methods. Each lens element corrected a specific type of aberration. However, just as one type of aberration was corrected, another was generated. The greatest feature of this lens is its aberration correction, achieved by putting it on a diet so to speak. Its diameter was reduced by simplifying each lens group. That is, the number of elements used in each group was kept to a minimum. This lens was refined just as animals are refined with evolution when, for example, a tail that is no longer needed gradually degenerates and is ultimately lost. That may be a bit of an exaggeration, but you get the point. Aspherical lens elements and ED glass were introduced to make this possible. The AF Zoom-Nikkor 28-200mm f/3.5-5.6G IF-ED effectively utilizes three lens elements with four aspherical surfaces, which allowed for fewer lens elements in groups two and four. Group four must usually be constructed with four to six elements, but we were able to construct it of just two elements, one convex and one concave, with aspherical surfaces. Two is the minimum number of elements required to correct chromatic aberration. By reducing the number of lens elements in each group, we were able to make the lens even much smaller than the target size. Then, contrary to the general design way of thinking, we were able to reduce each group's power. This increase in design flexibility made it possible for us to achieve even better optical performance, and achieve the world's smallest and lightest, truly all-purpose, 28-200mm high-power zoom lens. I think that heading into 2018, the AF Zoom-Nikkor 28-200mm f/3.5-5.6G IF-ED still holds the record for being the world's smallest and lightest high-power zoom lens.
V. Optical performance
Now let's take a look at the aberration characteristics of the AF Zoom-Nikkor 28-200mm f/3.5-5.6G IF-ED. We'll start at the 28mm maximum wide-angle position. Spherical aberration, which uses higher-order aberration generated by aspherical surfaces, is slightly under corrected. At the time of the lens's release, I think the amount of spherical aberration remaining was on the low end. I took particular care with regard to compensation for astigmatism and curvature of field. Astigmatic differences have been reduced as much as possible in both the sagittal and meridional planes for correction throughout 85% of the image height (around 85% of the entire frame). However, astigmatism gradually increases in higher portions of image height. Coma is also controlled quite well throughout 85% of the image height, but image quality suffers at the maximum image height with occurrence of both sagittal and meridional coma. There is mild mustache distortion of up to −2.4%. This is actually a relatively low number for a zoom lens at the wide-angle position. Overall, image quality is sufficiently good with shortcomings evident primarily in the four corners of the frame. Next let's consider point-image formation using a spot diagram. The lens performs quite well everywhere except the four corners of the frame. However, coma at the edges of the frame generates flare for signs of reduced sharpness.
And how does the lens perform at its mid-range focal length of 35mm? Aberration compensation performance is very similar to that achieved at the 28mm position, but coma, astigmatism, and curvature of field in the four corners are further corrected, almost as if the image were cropped to the 35mm angle of view. Distortion is at almost 0%. Users can expect quite good rendering at this focal length. The only real drawback is a slight increase in sagittal coma. Users may have to keep an eye on this when shooting at night.
Next let's take a look at rendering at the 50–135mm range of focal lengths. Within this range of focal lengths, rendering is quite consistent with little variation in aberration. With spherical aberration slightly over-corrected, rendering is reminiscent of Mr. Wakimoto's design characteristics. As for blur characteristics (bokeh), double-line blur may often occur in blurred background portions. However, by stopping down the aperture just half a stop, sharpness is greatly increased at and near the center of the frame. Astigmatism and curvature of field reflect design intent just as they did at other positions with astigmatic differences reduced as much as possible in both the sagittal and meridional planes for correction throughout 70% of the image height (around 70% of the entire frame). Distortion is a little high at +3–5%, for noticeable pincushion distortion. The degree of distortion exhibited is high enough to be problematic. Today, this distortion can be resolved with image processing, but at the time the lens was released, it was a real weak point.
Finally, let's see how the lens performs at the 200mm maximum telephoto position. Spherical aberration, which uses higher-order aberration generated by aspherical surfaces, is fully corrected with slightly complicated compensation. I think this lens would surely fall into the category of lenses with little remaining spherical aberration. Coma tends toward outer coma with a core that is surrounded by flare. Point images are affected by outer coma flare, but I think that optically speaking, three-dimensional rendering characteristics are quite good. Further, stopping down the aperture at this position would further increase the benefits of aberration compensation. Stopping down the aperture just half to one stop eliminates flare, and greatly increases sharpness, especially in portions at the center of the frame. Distortion is quite high at approximately 5%.
VI. 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 28mm position
f/3.5 maximum aperture
Slight flare at the center results in pleasant resolution and sharpness. The level of sharpness is fairly consistent from the center of the frame to the edges. However, the image deteriorates and flare increases at the extreme edges of the frame. Very little color bleed can be seen.
f/4 to f/5.6
Stopping down the aperture to f/4 reduces flare from the center of the frame to the edges. Both contrast and resolution are increased. Sharpness drops slightly, only at the extreme edges of the frame, but this issue is resolved by further stopping down the aperture to f/5.6.
f/8 to f/11
Even more consistent rendering is achieved throughout the entire frame. Most notably, contrast increases for quite pleasing image quality. 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/22
Resolution drops the more the aperture is stopped down. At f/22 especially, the effects of diffraction are visible and resolution drops.
Mid-range 35mm position
f/3.8 maximum aperture to f/4
There is less flare at the center of the frame than at the 28mm position, and sharpness is even greater. The good image range is wider than at 28mm, almost as if the image were cropped to the 35mm angle of view. However, flare occurs at the extreme edges of the frame. Very little color bleed can be seen.
f/5.6 to f/8
Stopping down the aperture to f/5.6 reduces flare from the center of the frame to the edges. Both contrast and resolution are increased. Sharpness drops slightly at the extreme edges of the frame, but this issue is resolved by further stopping down the aperture to f/8.
f/8 to f/11
Even more consistent rendering is achieved throughout the entire frame. Flare is eliminated everywhere except the extreme edges of the frame for sharp imaging.
f/16 to f/25
Resolution drops the more the aperture is stopped down. At f/22 to f/25 especially, the effects of diffraction are visible and resolution drops.
Mid-range 50mm position
f/4.5 maximum aperture
The center of the frame is sharp with very little flare. Some flare occurs at the edges, but resolution remains. Images exhibit a degree of sharpness sufficient for practical use. At this focal length as well, there is little color bleed.
f/5.6 to f/8
Stopping down the aperture to f/5.6 eliminates flare and increases contrast. The image deteriorates a little only at the extreme edges of the frame. However, this can be nearly completely resolved by stopping down the aperture to f/8. Contrast is good to the edges of the frame. Of all aperture settings, the best image quality is achieved at f/8.
f/11 to f/16
Consistent rendering is achieved throughout the entire frame. Contrast especially is greatly increased. At f/16 the effects of diffraction are slightly visible and resolution drops a little.
f/22 to f/25
Rendering is consistent, but resolution drops. The effects of diffraction are visible and resolution suffers.
Mid-range 85mm position
f/5.3 maximum aperture
Though resolution is fairly good from the center of the frame to the edges, the entire frame seems to be enveloped by flare. Rendering performance leaves a good impression as it is fairly consistent throughout the entire frame. Performance at this focal length seems suitable for portraits. At this focal length as well, there is little color bleed.
f/8 to f/11
Stopping down the aperture to f/8 eliminates flare and increases contrast. Contrast is good to the edges of the frame. Of all aperture settings, the best image quality is achieved at f/11.
f/16 to f/32
Rendering is consistent, but resolution gradually drops the more the aperture is stopped down. The effects of diffraction are visible and resolution suffers.
Mid-range 135mm position
f/5.6 maximum aperture
There is less flare than at the 85mm position, but resolution seems to have dropped. Rendering performance leaves a good impression as it is fairly consistent throughout the entire frame. At this focal length as well, there is little color bleed.
f/8 to f/11
Stopping down the aperture to f/8 increases sharpness. Further improvement is seen at f/11 with good contrast to the edges of the frame. However, some coma blur can be seen at the extreme edges of the frame. Stopping down the aperture to f/16 eliminates the blur.
f/16 to f/32
Rendering is consistent, but resolution gradually drops the more the aperture is stopped down. The effects of diffraction are visible and resolution suffers.
Maximum telephoto 200mm position
f/5.6 maximum aperture
Images are sharp and have good resolution at the center of the frame. There is also little flare. Resolution gradually drops farther away from the center. Resolution drops significantly at the extreme edges of the frame. At this focal length as well, there is little color bleed.
f/8 to f/16
At f/8 resolution seems even better. At f/11 even more consistent rendering is achieved throughout the entire frame. Of all aperture settings, the best image quality is achieved at f/11 to f/16. An aperture setting of f/11 is best for landscape photos.
f/22 to f/32
Even more consistent rendering is achieved throughout the entire frame, but resolution decreases as the aperture is stopped down. At f/22 to f/32 especially, the effects of diffraction are visible and resolution drops. If sharpness is the goal, the best results would likely be achieved at an aperture setting of around f/11 at all focal lengths (positions). I hope you now understand that this lens exhibits little variation in image quality with zooming.
Now let's confirm these rendering characteristics with some sample photos.
So that you may judge the characteristics of this lens for yourself, image sharpness has in no way been enhanced.
Sample 1 was captured at the wide-angle 28mm focal length with the aperture set to f/3.5 maximum aperture. Resolution is sufficient from the center of the frame to the edges, and there is no degradation, even at the extreme edges of the frame, for a level of image quality sufficient for practical use. A little flare tends to soften the image.
Sample 2 was captured at the mid-range 50mm focal length with the aperture set to f/4.5 maximum aperture. Just as at the maximum wide-angle position, image quality sufficient for practical use is maintained from the center of the frame to the edges. As focus is slightly in front of the model with this sample image, resolution in her face is just a little weak.
Sample 3 was captured at the mid-range 85mm focal length with the aperture set to f/5.3 maximum aperture. Just as at the maximum wide-angle position, image quality sufficient for practical use is maintained from the center of the frame to the edges. Again, with focus slightly in front of the model in sample 3, resolution in her face is just a little weak. There is little flare, and the degree of bokeh is neither notably good nor bad.
Sample 4 was captured at the telephoto 200mm focal length with the aperture set to f/5.6 maximum aperture. The image exhibits sufficient resolution from the center of the frame to the edges, and there is no degradation, even at the extreme edges of the frame, for a level of image quality sufficient for practical use. There is little flare, and the degree ofbokeh is neither notably good nor bad. With some scenes however, it may appear a little harsh.
VII. The old lens revival
Over the past few years, we have seen the revival of one old lens after another. I think that the primary reason for this is the fact that mirrorless cameras support mount adapters that allow users to play with a wide variety of lenses. Photographers are taking a new look at their old lenses, and rediscovering the soul that still resides in these once dead-stocked lenses. This is a wonderful thing. Many F-mount lenses especially have an incredibly long life if not discarded, enabling users to enjoy unique differences in their rendering characteristics.
Another reason for the revival of old lenses is the growth of the networked society. SNSs and websites can be viewed in an instant by just about anyone anywhere. Information can be shared immediately. People can share their thoughts—"That is a great lens!" or "I'd like to try this lens...". Lens rendering, bokeh, and three-dimensional reproduction characteristics are being discussed and debated somewhere every day. Even the AF Zoom-Nikkor 28-80mm f/3.3-5.6G covered in Tale 63 and the AF Zoom-Nikkor 28-200mm f/3.5-5.6G IF-ED discussed in this tale are considered two of the Nikkor Best 10 by a respected photographer and critic overseas. Another famous overseas critic has also shone a light on these lenses, extolling their compact size and excellent performance.
Over the past few years, I've been remembering approximately 25 lenses I designed in my 32-year career, and buying up those that I had previously failed to purchase. One of those was this AF Zoom-Nikkor 28-200mm f/3.5-5.6G IF-ED, which I only recently purchased. However, its price on the secondhand market was already starting to jump up at the time I purchased. Getting this lens to mass-production was more difficult than I ever expected. As a result, its release was also much later than expected. We were unlucky in that when the lens was finally released, a rival had just released a 28-300mm lens that prevented us from reaching target sales. Consequently, the number of used AF Zoom-Nikkor 28-200mm f/3.5-5.6G IF-ED lenses on the market is relatively low. It seems that is one reason why the lens is now so expensive.
I think this revival and popularity of old lenses will continue for the time being. I hope that this revival will take place not only in Japan but around the world, resulting in activating the photo culture so that the industry and hobbyists will continue to evolve.