NIKKOR - The Thousand and One Nights No.69
Medical-Nikkor 120mm f/4 IF
In Tale 69, we will take a look at the Medical-Nikkor 120mm f/4 IF, a lens that Nikon developed and released in response to demand from the medical industry. As a second-generation Medical lens, this lens could be considered a perfected macro Nikkor lens. Let's explore the secrets behind the Medical-Nikkor 120mm f/4 IF.
by Haruo Sato
I. The history and evolution of Medical-Nikkor lenses
Let's begin with an investigation into the birth of the Medical-Nikkor. At the time, Nikon's Instruments Division handled optical instruments and devices for the medical field. The Instruments Division had developed strong ties to the medical industry through its microscopes and other optical instruments, and therefore received many requests from that industry. With such strong ties to medicine and research, the company decided that a macro lens that could easily be used to accurately record surgeries and keep research records was needed. This macro lens should be effective for examining and documenting not only body cavities, but also the mouth. These considerations led to the start of development of the first-generation Medical-Nikkor Auto 200mm f/5.6. First, drawings for trial production were issued in January of 1962. Mass production began in May of the same year, and the lens was officially released in December 1962. In 1962, the 43-86mm zoom had yet to be released, and sales of the Nikon F were only in their third year. That means that this lens was conceived and designed very early in the history of the Nikon F mount system. The Medical-Nikkor Auto 200mm f/5.6 underwent two minor modifications. The first modification to the original model was made in 1972 with some cosmetic improvements to enable refinements to handling. The second modification was made in 1974 when multi-layer coating was applied. After that, the second generation of the lens was introduced with the release of the Medical-Nikkor 120mm f/4 IF.
II. Development history and the designer
Now let's look at the development history behind the subject of this Tale, the Medical-Nikkor 120mm f/4 IF. Yutaka Iizuka of Optical Designing Department at that time was in charge of designing the optics for the lens. There are no clear records stating when Iizuka began designing the lens, but I believe it had to have been in 1978 or 1979. Records do show that in 1979, drawings for trial production were issued with the focal length changed from 105 mm to 120 mm. The lens was originally to have a focal length of 105 mm. Why was it changed to 120 mm? At that time, 120 mm was certainly an unusual focal length. When I asked the designer himself about it, he told me that while he didn't remember all of the details, they had decided that 120 mm supported the optimal working distance. Priority was given to flexibility of use in the operating room, and that fundamental changes were applied to a lens they had already designed once. It was very close to New Year's Eve 1981 when the new Medical-Nikkor 120mm f/4 IF lens was finally released.
III. Micro and macro
As I also explained in Tale 25, there are clear differences between the photographic terms "micro" and "macro". Microphotography is reduction photography. Macrophotography indicates a type of photography with which the subject is recorded at its actual size (1 : 1) or larger. That is why Nikon's first close-up photographic lenses were known as Micro-Nikkor lenses. What about the Medical-Nikkor lens that is the subject of this Tale? An attachment could be used with this lens to achieve a reproduction ratio of 1 : 1 (life size) and larger (up to 2x life size). The lens exceeds the micro range with macro range photographic capability. Nikon named the lens "Medical-Nikkor" with a very great desire for this lens to be useful in medical fields. If another name for the lens, developed with such high ambitions, had been permitted, I think I would have liked to see it called a Macro-Nikkor lens.
IV. Lens construction and characteristics
First, take a look at the cross-section of the Medical-Nikkor 120mm f/4 IF (Figure 1). Please forgive me if the following is quite technical.
The lens Iizuka designed was an extremely unique type of lens. The lens element nearest the subject (farthest from the mount) in Figure 1 is a dedicated attachment lens that enabled shooting at a reproduction ratio as high as double life size (2x). It is a removable achromatic doublet or close-up lens. Behind it is the very original Medical-Nikkor itself. The lens is constructed with a convex-concave-convex three-group structure. Those who see a telephoto zoom structure here are quite sharp. In actuality, this structure is a new type of macro lens structure that Iizuka invented. In fact, the way magnification changes using a zoom lens' variator lens group and that with focusing are physically very similar. However, the fundamental difference lies in whether the distance between the object and the imaging surface changes. If focal point compensation is not performed with zoom lenses, the distance between the object and the imaging surface will change and, as a consequence, the change in magnification varies focus position. Iizuka achieved a unique macro lens by modifying the telephoto zoom structure so that the zooming group performs the focusing. I asked Iizuka himself about the secrets behind the development of this lens. It seems that he was designing a telephoto zoom lens just prior to being assigned the lens that would become the Medical-Nikkor 120mm f/4 IF. There is no doubt that Iizuka's head was full of ideas for improving the performance of telephoto zoom lenses. It was then that he came upon the design for this Medical-Nikkor. Iizuka shone. His idea was to make slight changes to a telephoto zoom's master lens group and use the concave group for focusing as with super-telephoto lens' internal focusing. And, indeed, this proved very effective.
V. Using the Medical-Nikkor 120mm f/4 IF
Using the Medical-Nikkor 120mm f/4 IF requires some tricks and practice. One of the major differences from most other lenses is that photography using the ringlight flash built into the object end of the lens is presumed. This ringlight flash had a fixed guide number. Therefore, at the same ISO sensitivity, the aperture value was determined in accordance with the reproduction ratio (object distance). With the attachment lens removed, the object distance of this lens was 1 : 11 (object distance: 1.6 m) to 1 : 1 (life size; object distance: 0.35 m). However, the aperture was fixed at f/4 with a reproduction ratio of 1 : 11, and at f/32 with a reproduction ratio of 1 : 1 (life size). When the 0.8-2x attachment lens was attached, the aperture value was fixed at f/32. A small optical system was positioned near the mount to include the reproduction ratio within the frame. When I tried out the lens, I was surprised by the working distance. I wondered why the working distance had to be so long with this lens. I looked at the user's manual for the first-generation Medical-Nikkor and understood immediately. Medical-Nikkor lenses were designed for use in an operating room from a position that would not interfere with the surgeon. Clearly these lenses were strictly developed to address real needs of the target users. This is the attitude we should never forget. In addition, I tried out the lens without using the built-in ringlight flash, in part because I was unable to acquire the proper power source. When used with a digital camera, shutter speed and ISO sensitivity must be adjusted to achieve the desired exposure. As an older lens, using the Medical-Nikkor comes with a number of restrictions. However, it certainly has its charms as a macro lens.
VI. Design values and rendering characteristics
If we look at design values, one of the first things we notice is that there is little distortion throughout the entire frame. This does not change when the attachment lens is used with distortion measuring just ±1% or less throughout the entire frame. In addition, chromatic aberration, and especially lateral chromatic aberration, is well compensated throughout the entire frame. As, with these optics, the aperture is stopped down except when shooting at low reproduction ratios, it is likely that they are designed to control as much as possible aberrations that are not corrected by stopping down the aperture. At or around 1 : 11, spherical aberration and axial chromatic aberration are both well suppressed. There tends to be insufficient compensation for curvature of field with shooting at low reproduction ratios, but it is generally well compensated at higher reproduction ratios. Next let's look at spot diagrams. Point image reproduction is good at with shooting at low reproduction ratios, and background blur (bokeh) tends to be quite beautiful. As the reproduction ratio increases, coma that corrupts point image formation occurs. However, as the aperture is actually stopped down to f/11-f/32, no major problems occur. The same goes for MTF. At low reproduction ratios, 10- and 20-lines/mm MTFs consistently show good values. What's more, sufficient sharpness is exhibited with shooting at high reproduction ratios and when the attachment lens is used, even considering a 5- to 10-lines/mm MTF as the basis. Further, as the aperture is stopped down to f/32, the need to consider geometrical aberrations is eliminated. However, when a digital camera, and especially a newer one released in recent years, is used, there is a concern that image quality will be reduced, perhaps significantly, due to the effects of diffraction.
VII. Actual performance and sample images
Next, let's look at imaging results achieved with capture of two-dimensional charts at each reproduction ratio. As I noted before, the aperture value is fixed according to object distance. Evaluations are subjective, and based on individual preferences. Please keep in mind that my opinions on sample images and evaluations below are for reference purposes.
Around 1 : 11 (or 1/11x; f/4)
Resolution is relatively good, but a certain amount of chromatic aberration is noticeable throughout the frame. The amount of color shift increases at the edges of the frame. Contrast is a little low. Direct light from the flash generally results in an image with strong contrast. However, as the optics built into this lens cause a slight reduction in contrast that offsets the strong contrast caused by the flash, the resulting tones are just right. I don't know if this was an intentional objective with design, but I would be astonished if it were considered with design that time.
Around 1 : 8 (or 1/8x; f/5.6)
Contrast increases, chromatic aberration is corrected, and coloring is minimized. Resolution is also increased.
Around 1 : 4 (or 1/4x; f/11)
Contrast increases, and the effects of chromatic aberration are not visible. Resolution decreases slightly.
Around 1 : 2 (or 1/2x; f/11)
There is sufficient resolution and contrast. Issues such as color shift are nearly completely eliminated.
Around 1 : 1 (or life size; f/32)
Contrast is good. Issues such as color shift are nearly completely eliminated. However, the image becomes dull and resolution drops, perhaps due to the effects of diffraction.
The attachment lens was used with the following.
Around 0.8 : 1 (or 0.8x; f/32)
Contrast is good. However, the image becomes dull and resolution drops slightly, perhaps due to the effects of diffraction.
Around 1.5 : 1 (or 1.5x; f/32)
A certain degree of resolution remains at and around the center of the frame, but it drops at the edges of the frame. There is no color shift. However, the image becomes dull and resolution drops slightly, perhaps due to the effects of diffraction.
Around 2 : 1 (or 2x; f/32)
There is no color shift, but sharpness is reduced. Resolution at the edges of the frame is poor. Contrast is also low.
Now let's confirm these rendering characteristics with some sample photos.
These images are very near the state at which they came out of the camera. Image sharpness has in no way been enhanced so that you may judge the characteristics of this lens for yourself. Further, the built-in flash was not used to capture these sample images. However, there was none of the stress that occurs regarding image quality with shooting these sample images.
Sample 1 was captured at the 1 : 5 reproduction ratio and the maximum aperture of f/8 under natural light. Given the range of this lens, 1 : 5 is a relatively low reproduction ratio. As no artificial light source was used, shooting was entirely manual. After some trial and error, I settled on ISO 400 and a shutter speed of 1/250 s. Focus is sharp and clear, but contrast is a little low. The blossoms were lit directly by the sun, but did not result in contrast that was too harsh. In fact, I am quite pleased with the results. The gentle bokeh is attractive. Despite containing many aspects that are likely to cause double-line blur, beautiful bokeh is reproduced without being affected by those aspects at all.
Sample 2 was captured at the 1 : 1.5 reproduction ratio and the maximum aperture of f/22. As the aperture value is determined by the reproduction ratio with this lens, shooting without the built-in flash can prove to be difficult. This image exhibits sufficient sharpness, but diffraction causes an unfortunate drop in resolution. Despite being captured under natural lighting that can result in strong contrast, just the right amount of compression has been achieved for pleasant contrast. As with the sample image 1, pleasing bokeh has been achieved. When shooting photos like this at short distances that achieve a shallow depth, no more than one of several hundredths (1/X00ths) of the entire frame is in focus. Therefore, the sharpness of the in-focus portion is very important, as is three-dimensional rendering performance (rendering characteristics). The characteristics exhibited in this photo surely make this a good lens.
Sample 3 was captured at the 1 : 1 (life size) reproduction ratio using the attachment lens, and the maximum aperture of f/32. It was captured under soft indoor lighting. Some grain is visible as sensitivity was increased to ISO 1600. This image exhibits sufficient sharpness, but diffraction causes an unfortunate drop in resolution. Contrast is a little low, but is quite appropriate for the subject. The bokeh is also pleasing with this image.
Sample 4 was captured at the 2 : 1 (2x) reproduction ratio using the attachment lens, and f/32. If we look at the shadows in the marbles and the veins in the leaf, it is clear that this lens provides a level of sharpness that stands up to practical use. However, as I've said so many times before, discussion of geometrical aberrations and such are basically meaningless at f/32. The effects of diffraction are great and have a very negative impact on resolution. This really is too bad.
VIII. Yutaka Iizuka
When I first started at Nikon, I was assigned to the development section of Optics Designing Department. Iizuka, who had a vast amount of experience in optics, was in that section at that time. My first impression was of an extremely slim man with a frank and bright personality, and a dynamic laugh. Iizuka designed a variety of optical systems. He was particularly good at zoom-lens design such as the Series E 75-150mm f/3.5 that was the topic of Tale 42 by Kouichi Ohshita. As a matter of fact, the arrangement of Medical-Nikkor 120mm f/4 IF lens elements is somehow similar to that of the Series E 75-150mm f/3.5. It seems that, together with Yoshinori Hamanishi, Iizuka perfected the lens arrangement of the afocal zoom invented by Souichi Nakamura. That's just how knowledgeable he was about zoom lenses. My ties with others in the development section of Optics Designing Department to which I was first assigned are strong, and I am still indebted to them. Looking back, I remember that Iizuka and I worked together for only a short time. However, he taught me about paraxial theory, which is at the root of optical design, as well as how to read change diagrams. The things he taught me helped to form the foundation of my own basic knowledge of optics. After his retirement, Iizuka formed his own optical design company, where he continues to work. I hope that he will forever activate the industry through his optical designs.