NIKKOR - The Thousand and One Nights No.74
The evolution of the normal Micro lens
The AI AF Micro-Nikkor 60mm f/2.8S
In this tale, we'll discuss the AI AF Micro-Nikkor 60mm f/2.8S, which was developed at the same time as the 105mm AF Micro-Nikkor covered in Tale 72.
by Kouichi Ohshita
I. The AI AF Micro-Nikkor 55mm f/2.8S
Haruo Sato has already covered Micro lens development through the manual focus Ai Micro-Nikkor 55mm f/2.8S in Tales 25 and 26, so I'll limit my coverage to the initial transition to AF and beyond.
From the initial planning stages of the F-501/N2020, which was released in 1986, it was decided that an AF Micro lens would be released at the same time as the camera. Further, as I noted in Tale 72, the primary objective for this Micro lens was to increase the maximum reproduction ratio to 1:1 (life size). Up against a tight deadline, Yoshinori Hamanishi, who was in charge of Micro lenses at the time, thought that there was no time to redesign the lens from scratch and verify trial production. Therefore, he decided to see if he could adopt optics from the existing Ai Micro-Nikkor 55mm f/2.8S and modify the floating mechanism to achieve a life-size reproduction ratio. In addition to aberration compensation, Hamanishi struggled with two other issues. The first was an issue brought up in Tale 72: the amount of movement required of lens elements. As movement of a conventional 1:2 lens must be doubled to achieve a 1:1 (life size) reproduction ratio, the basic assumption was that the new AF lens would have to support twice the amount of movement as the manual focus lens. AF speed decreases as the amount of lens movement increases.
The second issue was whether focus could be achieved using AF with the amount of movement required for the 1:1 reproduction ratio. When the entire lens group moves, the distance between the subject and the focal plane (shooting distance) is least at a reproduction ratio of 1:1. This makes for an interesting characteristic in that the distance between the subject and the focal plane must be increased in order to achieve reproduction ratios greater than 1:1. On the other hand, autofocusing demands that the shooting distance be gradually decreased with the transition from infinity to close-up. This is easier to grasp if you think of yourself with a camera in your hands. You mount the camera on a tripod with the distance between the subject and camera (shooting distance) fixed. When you press the shutter-release button halfway, the camera begins to focus (autofocusing). When you want to make the subject larger (take up a larger portion of the frame), you naturally move closer to it. When a floating mechanism is used, lens focal length and specifications change with shooting distance. While controlling these changes, focus must be designed so that the shooting distance always decreases until a 1:1 (life size) reproduction ratio is achieved.
Ultimately, the best design in terms of resolving these two issues and maximizing performance was achieved with the AI AF Micro-Nikkor 55mm f/2.8S.
II. Remaining issues
The AI AF Micro-Nikkor 55mm f/2.8S was released at the end of 1986, a little behind the F-501/N2020. However, it still exhibited some issues that had to be addressed. The first was the amount of movement required of lens elements. As this type of lens employed an all-group shifting system and a floating mechanism, the first group moved more than 60 mm. Of course, this made AF operation slow in the macro range. In terms of performance, the 1:2 reproduction ratio was extended to 1:1 by only redesigning the floating trajectory. As a result, imaging performance at the edges of the frame in the macro range did not meet Hamanishi's expectations. Spherical and chromatic aberrations were effectively compensated, and performance from the center of the frame to mid ranges was good. However, compensation to mitigate astigmatism and changes in field curvature in the extreme corners of the frame was not very effective. That is because the floating trajectory had to be shifted because of the two restrictions mentioned above: as much reduction of the amount of lens-element movement as possible, and the fact that shooting distance is always reduced with the transition from infinity to 1:1.
While I'm sure Hamanishi felt a sense of accomplishment at what he had achieved, he likely also felt some disappointment as well.
III. The AI AF Micro-Nikkor 60mm f/2.8S
An opportunity to address remaining issues soon presented itself. The AF Nikkor lens released at the same time as the F-501/N2020 was not very popular. Users complained that the narrow manual focusing ring was difficult to operate, so a new model with a modified exterior was planned. It was decided that a normal Micro lens with a modified exterior and completely new optics would be released at the same time as a 105mm Micro lens.
Hamanishi immediately began designing with Keiji Moriyama, designer of the 105mm Micro lens introduced in Tale 72. Hamanishi already had an idea of what his final design would look like. One is to reduce the amount of movement required by adding a teleconverter behind a Gauss structure just as the AI AF Micro-Nikkor 105mm f/2.8S. Another is to take advantage of the added design flexibility to increase performance at close distances. However, his approach had issues as well. One was backfocus restrictions. With the 105mm lens, it was easy to secure space for the teleconverter between the main Gauss-type structure and the focal plane. Hamanishi predicted that this would be difficult with a normal lens. With his first design attempt, he was unable to both preserve backfocus and achieve the necessary level of performance with the focal length of 55mm. He decided to increase the focal length to 60mm.
However, that does not mean that he solved his problem. Another issue was that the teleconverter magnified master lens aberration. A teleconverter has the advantages of controlling the amount of movement needed, and increasing flexibility that allows for greater performance, but in principle, it also increases master lens aberration. Hamanishi manipulated teleconverter magnification and the trajectory with which the master lens moves to find ways to achieve a structure that moved as little as possible, yet provided the best performance possible, finally arriving at that shown in Fig.1.
Fig. 1 is a cross-section of the AI AF Micro-Nikkor 60mm f/2.8S. It is designed with a modified Gauss-type structure comprised of six elements in five groups with a two-element teleconverter behind this main structure. Three elements in front of the aperture and three elements behind the aperture move independently to focus from close-up and 1:1 to infinity. Adding a teleconverter behind the main Gauss-type structure made for much less movement than was necessary with the AI AF Micro-Nikkor 55mm f/2.8S. The lens barrel mechanism was also quite elaborate. As you can see from Photos 2 (lens focused at infinity) and 3 (lens focused at minimum focus distance), the structure was designed with two inner barrels inside the outer barrel. This design allowed for the large amount of movement required of Micro lenses, as well as a sharp appearance when focused at the minimum focus position. This barrel was designed by Kunihiro Fukino. My younger self used to gaze in rapture upon this design reminiscent of a bamboo shoot. I thought it looked so good. Multi-barrel structures like this have often been used for the mechanisms that extend the zoom lenses built into compact cameras, but at the time, this was quite an innovative mechanism.
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 FTZ mount adapter.
Sample 1 is a photo of the southern sky as summer changes to fall, and was highly influenced by light pollution. It is a composite of 39 exposures with the same composition that was processed to significantly increase contrast and reduce noise. The bright star in the bottom right corner of the frame is Altair in the Aquila constellation, and the small, sort of diamond-shaped constellation at the center of the frame is the Delphinus constellation. To the left of that is the Equuleus constellation, at center right is the Sagitta constellation, and to the right above that is the Vulpecula constellation. With the brighter Milky Way shown at right in the frame, I think this photo is a good representation of a starlit early-autumn sky, clearly showing how the number of stars begins to diminish just about at the Delphinus constellation.
At maximum aperture, this lens exhibits some faint axial chromatic aberration at the center of the frame, and a little sagittal coma flare at the edges of the frame. However, stopping down the aperture to f/4 produces extremely sharp images throughout most of the frame (only the extreme corners of the image are less sharp) as evidenced by this photo. Stars at or near the middle of the frame are incredibly sharp. When viewed at 100%, even the shapes of relatively larger nebulas and star clusters like M15, M27, and M17 can be seen. However, one of the drawbacks of this lens can be seen in the extreme corners of the frame where stars have been distorted to a fat triangular shape. On the other hand, the incredible abundance of peripheral illumination from infinity to close-up shooting at 1:1 is one improvement over the 55mm Micro lens. I felt no need to flatten even Sample 1, and the abundance of peripheral illumination acquired at maximum aperture, also indicated in following sample images, is clear.
Sample 2 is a photo of autumn leaves captured with the aperture stopped down to f/8. Stopping down the aperture to f/8 enables sharp rendering all the way to the extreme corners of the frame, but the background is slightly blurred. Keep in mind when shooting that depth of field is shallower with a 60mm lens than it is with a 50mm lens.
Sample 3 is a photo of cosmoses captured at maximum aperture. You can see how the petals gradually and smoothly become more blurred the farther away from the center of the flower they are. However, at medium distances like that expressed at the bottom of the frame, there is a slight edge to the blur (bokeh). As this photo was taken in the shade with little contrast, it is not very noticeable, but if the leaves and stems had been lit by sunlight, the bokeh would likely be somewhat distracting.
Sample 4 is a close-up of maple leaves captured at f/5.6. By stopping down the aperture, the borders visible in edges of blurred portions in Sample 3 have been completely eliminated. However, the shape of the seven straight iris diaphragm blades is noticeable. Some users may not like this, especially in backgrounds exhibiting sharp contrast. One of the advantages of this lens that must be pointed out however, is the precise and gentle rendering of fine lines in portions of the image that are in focus.
Sample 5 is a 1:1 (life size) photo of the center of an agate plate captured at the lens' minimum focus distance. This photo was taken at maximum aperture, but the finest details of crystal structure are clearly rendered all the way to the edges of the frame. If we look very closely, we see that some color bleed (axial chromatic aberration) is visible, but it does not affect resolution. The yellow edges visible around crystals at the center of the frame are the crystals themselves, and not the effect of aberration.
In Tale 72, I mentioned just how precise focus, the subject, and camera settings must be for 1:1 (life size) photography. When photographing a flat subject like this, one technique commonly used is squaring. That is, ensuring that the surface of the subject is perfectly parallel to the focal plane. A simple means of achieving this is by using a mirror. Place a mirror in contact with the object as demonstrated in Fig. 2. Adjust camera angle and position so that when you look through the viewfinder and see the camera and lens reflected in the mirror, the center of the lens is positioned at the center of the viewfinder display. Lock the camera in this position. Now remove the mirror. The object is "squared" with the camera/focal plane. This method is used to adjust a variety of optical devices, as it enables relatively high-precision positioning using just a mirror. Try it for yourself!
Finally, Sample 6 is a photo of a cyclamen captured at maximum aperture and the 1:1 (life size) reproduction ratio. The pistil and stamen, on which focus was acquired, are sharply and clearly rendered, but the flower petals melt smoothly into the background. As I also explained in Tale 72, depth of field at the 1:1 (life size) reproduction ratio is equal to (circle of confusion diameter) x (aperture value), and therefore not dependent on focal length. However, as the amount of background blur depends upon focal length, edges of blurred portions in this sample are relatively clear compared to those captured with a 105mm lens. I hope that those who own both a normal Micro lens and a telephoto Micro lens try to see this difference in rendering for themselves. I myself have tried some lenses including one of my own favorites, the AI AF Zoom Micro-Nikkor 70-180mm F4.5-5.6D, and felt like the 60mm angle of view was better suited for photographing a plant like cyclamen whose flowers grow densely.
V. A popular choice for more than 30 years
This lens, which was released in 1989, before the AI AF Micro-Nikkor 105mm f/2.8S, was later equipped with an absolute distance encoder and released as the AI AF Micro-Nikkor 60mm f/2.8D in 1993. That lens was very popular and became the new standard amongst normal Micro lenses. Further improvements were made to the lens, and it was released as the AF-S Micro NIKKOR 60mm f/2.8G ED in 2008. 2020 marks more than 30 years that this lens has been a popular choice amongst photographers of all levels. The AF-S Micro NIKKOR 60mm f/2.8G ED is equipped with two aspherical lenses and one ED glass lens that do an incredible job of correcting axial chromatic aberration, and spherical aberration and coma especially with shooting at close distances. If you are looking to purchase a new Micro lens, I would like to recommend this lens to you, but the older 60mm f/2.8D still seems to have popularity as it has been produced even after the new 60mm f/2.8G ED was released. Its appeal certainly comes from gentle rendering characteristics backed up by an abundance of peripheral illumination. However, I wonder if I am the only one who is also attracted to the mechanical beauty of the lens when extended for 1:1 (life size) photography.
NIKKOR - The Thousand and One Nights
The history of Nikon cameras is also that of NIKKOR lenses. This serial story features fascinating tales of lens design and manufacture.