NIKKOR - The Thousand and One Nights No.70

—The Battle with Chromatic Aberration and the AI Nikkor 300mm f/4.5 S —

AI NIKKOR 300mm f/4.5 S

NIKKOR The Thousand and One Nights have covered some of the first popular telephoto lenses for SLR cameras, including the Nikkor-Q Auto 200mm f/4 (Tale 48) and the Nikkor-Q Auto 400mm f/4.5 (Tale 50). I would now like to conclude with coverage of 300mm f/4.5 lenses.

by Kouichi Ohshita

I. Nikkor-P Auto 300mm f/4.5

The Nikkor-P Auto 300mm f/4.5 was the first 300mm lens released for use with the Nikon F Series. It was designed by a man who plays a regular role in this series, Saburo Murakami. The design was completed in 1964. The lens then went into trial production, and was ultimately released in the same year. It was released at about the same time our super-telephoto lens series using focusing units were released. Fig. 1 is a cross section of this lens. It is a common structure used for telephoto lenses with a front lens group comprised of convex-convex-concave elements and a rear group with convex-concave elements. As the total length of this type of telephoto lens (from the vertex of the front element to the focal point) can be made shorter than the focal length, this structure is advantageous for making lenses with long focal lengths more compact. This telephoto lens was designed to have an even shorter total length with a concave element also positioned as the last of the three elements of the front lens group.

Fig. 1. Nikkor-P Auto 300mm f/4.5 cross section

The desire to make this lens so much more compact was based on the differences in characteristics from super-telephoto lenses with focal lengths of 400mm and longer. Super-telephoto lenses that use a focusing unit were developed with the assumption that tripods will be used to support their length and weight. This 300mm lens, however, needed to be designed to have as short a total length as possible so that it would offer good handling, as it was originally planned to be just small and light enough for hand-held shooting. These efforts resulted in a compact 300mm lens with a "telephoto ratio" (lens total length divided by the focal length) of approximately 0.8.

II. The battle with chromatic aberration

Designing telephoto lenses is a battle with chromatic aberration. We were at the greatest disadvantage in this battle with this 300mm lens. Chromatic aberration increases along with the focal length of the lens. This is easiest to understand if we imagine that it is proportional to the length of the lens. If the length of the lens is doubled—the focal length is doubled—the amount of chromatic aberration is doubled at the same time. Further, the amount of chromatic aberration also increases with the adoption of the common telephoto structure used for the Nikkor-P Auto 300mm f/4.5 to achieve a shorter total length. The concave lens in the rear group reduces the total length of the lens and achieves a consistently flat image plane, but it also works to increase all types of aberrations, including chromatic aberration, generated by the first group of elements.

The telephoto structure was adopted for the 200mm lens introduced in Tale 48, but with a "telephoto ratio" very close to 1, the total length of the 200mm lens was not forcibly reduced. In addition, in the case of the 400mm lens with an Ernostar structure introduced in Tale 50, it was designed to extend the total length of the lens so that the aperture position could be the same as that of the 600mm lens. Therefore, if we compare the 200mm, 300mm, and 400mm lenses, we find that the great reduction in the total length of the 300mm made it the worst in terms of chromatic aberration.

It was Tadashi Takahashi that accepted the challenge to improve the performance of this 300mm lens. Along with Wakimoto, Isshiki, and Higuchi, Takahashi was one of the pioneers of NIKKOR development. He participated in the design of many industrial NIKKOR lenses, as well as the original EL Nikkor 80mm f/5.6 introduced in Tale 64.

The objective was to improve the design of the lens without increasing its total length or modifying its structure too much. Significant changes to the structure of the lens would require a complete redesign of the lens barrel. Therefore, it was best that the structure be changed as little as possible. Takahashi tackled the goal of reducing chromatic aberration by changing the combinations of glass materials used. His design was completed and trial production initiated in 1966. However, his success was marginal. He had achieved his goal of improving performance at the edges of the frame with significant reduction of lateral chromatic aberration, but there was no increase in performance at the center of the frame. In fact, performance there had even dropped a little. In short, he had merely changed the balance of chromatic aberration rather than achieving any significant improvement. Finding that it was too difficult to achieve major improvements while keeping the five-element structure, Takahashi decided to modify the lens structure. In the fall of 1967, one year after his first attempt went into trial production, design data for his latest design was completed and trial production initiated.

III. Nikkor-H Auto 300mm f/4.5

This new trial production lens showed significant increases in performance that only began with reductions in chromatic aberration. It was ultimately released as the Nikkor-H Auto 300mm f/4.5 in 1969. Six years later, in 1975, it was reborn as the new Nikkor 300mm f/4.5 with a redesigned exterior. In 1977, the final incarnation of the lens as the AI Nikkor 300mm f/4.5 S was released, ultimately making it a very popular lens among users over a long period of time. During this time, coatings were improved and the lens barrel was redesigned, auto indexing was added, and the minimum focusing distance was reduced (from 4 to 3.5 m), all with very little structural change to the optics.

Nikkor-H Auto 300mm f/4.5 (top) and AI Nikkor 300mm f/4.5 S (bottom)
Fig. 2. AI Nikkor 300mm f/4.5 S cross section

Fig. 2 shows how this lens is constructed. If we compare it to the Nikkor-P Auto 300mm f/4.5 in Fig. 1, we see that the structure of the rear groups looks the same, but that of the front groups differs greatly. The front group of the Nikkor-P Auto 300mm f/4.5 was constructed using three convex-convex-concave lens elements, but the front group of the AI Nikkor 300mm f/4.5 S (as well as the Nikkor-H Auto) appears quite different with a four-element structure made up of a convex-concave doublet, a convex element, and a concave element. However, if we look closely, we see that the only difference was the adoption of a doublet for the first lens element, and that basically the structure was changed very little.

So just how did Takahashi increase performance? He took another look at the glass materials used. Common practice for compensating for chromatic aberration is to use crown glass for convex lens elements and flint glass or dense flint glass with a higher refractive index for concave elements. Takahashi began by considering replacing the concave element in the front group with one made of short flint glass, which provides effective chromatic aberration compensation. This type of glass was the best option available for chromatic aberration compensation at a time when ED glass had yet to be developed. However, short flint comes with its own drawbacks. It has a lower refractive index than dense flint glass, and with color dispersion similar to that of crown glass, spherical aberration increases. By adopting a doublet for the first element so that two concave elements could be used in the first group, Takahashi was successful in both reducing chromatic aberration and compensating for spherical aberration.

IV. Lens rendering

As always, let's take a look at this lens' rendering characteristics with sample images. The sample images for this Tale were captured using the Z 6 full-frame mirrorless camera and FT-Z mount adapter. Thanks to the high-definition electronic viewfinder built into the Z 6, it was even easier to achieve accurate and precise focus than it is with an SLR camera, which made using this manual focus lens to capture sample photos extremely easy.

Sample 1 is a photo of the center portion of the well-known Orion constellation. As this image was made in less-than-optimal conditions highly influenced by light pollution, it is a composite of 11 exposures with the same composition that was processed to significantly increase contrast and reduce noise, and for which a flat image was used to compensate for peripheral illumination falloff. Therefore, this sample image does not exhibit true performance with regard to peripheral illumination falloff. In addition, as significant processing was used to increase contrast, the resulting image shows a very noticeable amount of flare and chromatic aberration. So let's keep these things in mind when considering this sample. Even as it performs a great deal of processing, images captured with the Z 6 exhibit little noise. In addition, although no filter was used, differences in the colors in IC 434, which highlights the Horsehead Nebula, and those in NGC 2024, the Flame Nebula, are well rendered, which seems to make the Z 6 a good camera for capturing star photos.

First off, the first thing that surprised me was probably the lack of noticeable axial chromatic aberration. Normally, when a lens not made with ED glass is used to photograph bright stars, those bright stars end up with bluish purple halos extending from around the stars. However, even at the maximum aperture of f/4.5, you can see that this sample image exhibits no noticeable coloring around the stars. If we enlarge our view of the image and look very closely, we see that the light green coloring at and near the center of the frame, as well as the reddish purple coloring that begins at frame edges and moves toward the center of the frame, is effectively minimized. This is due to the skillful balancing of aberration to ensure a lack of noticeable coloring on the focal plane. However, this has its drawbacks. In this image, you can see that in general, individual stars are not sharply rendered. Further, stars at the edges of the frame are slightly distorted as a result of astigmatism.

Note the hourglass-shaped halos around the bright stars at the top of the frame. These are shafts of light created by the diffraction of light according to the shape of lens vignetting. The principle here is the same that causes shafts of light passing through a polygon-shaped aperture to radiate. The image prior to contrast enhancement exhibited some very light flare, but not enough to be noticeable in common night landscapes and the like.

Sample 2 is a photo of the "Fuji diamond", which is an optical effect created where the summit of Mt. Fuji and the sun appear to meet, captured from within the greater metropolitan area. As I was only able to capture this image from between tall buildings, the sun is not directly above the summit of the mountain. I began shooting as soon as the sun appeared from behind the building at left, but there was none of the ghost that normally accompanies strong sunlight. Part of that is due to the application of a multi-layer coating to lens elements. However, I offer one precaution. When photographing sunset scenes like this, do not attach a lens protector to the front of the lens. Doing so results in multiple reflections between the protector and the filter near the surface of the sensor, regardless of the structure of the photographic lens. This causes ghost to occur point-symmetrically from the sun with respect to the center of the frame. I stopped down the aperture to f/8 for this sample, but the edges around Mt. Fuji and the buildings do not seem especially sharp. This is the result of remaining chromatic aberration. While stopping down the aperture dramatically reduces spherical aberration and coma, it does little to reduce axial chromatic aberration. Further, if we look very closely at the edges of the buildings and the antennas, we see that lateral chromatic aberration is visible. Stopping down the aperture does not reduce lateral chromatic aberration.

Sample 1
Z 6 + FTZ + AI Nikkor 300mm f/4.5 S at maximum aperture, 15 s, ISO 1600; guide photography using an equatorial mount; processed with Capture NX-D, 11 exposures combined, processing to flatten
Sample 2
Z 6 + FTZ + AI Nikkor 300mm f/4.5 S at f/8, 1/4000 s, ISO 100; processed with Capture NX-D

Sample 3 is a photo of a Kawazu cherry in bloom in a nearby park. This type of flowering cherry blooms early and has become quite common. I focused on a blossom slightly above the center of the frame. I'd like to point out several petals throughout the frame. While there is little coloring around the edges of the petals on which I focused, red borders are visible around slightly blurred petals in the foreground and yellow green borders can be seen around blurred petals in the background. While no chromatic aberration is visible in portions of the frame that are in focus, it is noticeable in defocus portions.

In addition, you can see that the background is slightly influenced by flare due to insufficient spherical aberration compensation. As a result, background blur is relatively gentle, making this lens one that exhibits pleasing background bokeh. However, as the edges of the blurred elements are fringed by a yellow green border, these edges may become quite noticeable depending upon the scene's color scheme. On the other hand, the red fringe noticeable around blurred elements in the foreground may be a little harsh.

Sample 4 is a photo of a black-headed gull at Shinobazu pond. As chromatic aberration is noticeable along high-contrast edges on this nearly all-white bird, this is a subject for which this lens is not particularly well suited. A closer look reveals a red fringe around the bird's slightly out-of-focus tail, but color fringing doesn't seem to be a problem in in-focus portions of the frame. Further, as this sample was captured at maximum aperture, the background is very blurry. With this much blurring, coloring around blur is no longer noticeable. With the sky in the background at the top of the frame, some peripheral illumination falloff can be confirmed, but I don't think it's significant enough to be an issue.

Sample 3
Z 6 + FTZ + AI Nikkor 300mm f/4.5 S at maximum aperture, 1/320 s, ISO 280; processed with Capture NX-D
Sample 4
Z 6 + FTZ + AI Nikko 300mm f/4.5 S at maximum aperture, 1/800 s, ISO 100; processed with Capture NX-D

V. 300mm f/4.5 specs

300mm f/4.5 specs provide the highest possible brightness for a 72mm attachment size. Some of you may remember from previous Tales in this series that until a certain time, all lenses for the Nikon F Series were developed with one of only two attachment sizes, 52mm and 72mm. As I began refreshing my knowledge of the 300mm f/4.5 lenses in preparation for writing this Tale 70, I assumed that the f/4.5 specification was chosen simply in order to achieve a 72mm attachment size. However, that doesn't seem to have been the case. As I looked back through the oldest design reports, something in one of those reports caught my eye. I discovered design data for a 300mm f/4.5 lens with a telephoto structure from 1940. The designer was Saburo Murakami. Murakami must have remembered this design proposal while designing the 300mm lens for the Nikon F Series in the 1960s. I think Murakami had, in his own mind, decided on the maximum aperture of f/4.5 for the 300mm lens from the beginning.

In this day in age, when the use of ED glass is commonplace, I honestly didn't expect much in terms of rendering from the lens. As I captured the sample photos, however, I was surprised to find that while edges don't have the same sharpness produced by modern lenses, there was very little color fringing. This rendering performance is probably why the lens continued to be manufactured even after release of the 300mm f/4.5 ED, and was popular among users for a long time. The AI Nikkor 300mm f/4.5 S was a lens achieved through the designer's passion to reduce the chromatic aberration so common with telephoto lenses in an age when ED glass did not yet exist.