NIKKOR - The Thousand and One Nights No.84
The first line of fast wide-angle lenses for the Nikon F
AI Nikkor 35mm f/2S
In tonight's tale, we'll look at the AI Nikkor 35mm f/2S. With the same basic optical structure as the Nikkor Auto 35mm f/2, the first 35mm f/2 lens for the Nikon F, this lens was a super long-selling lens.
by Kouichi Ohshita
Nikkor Auto 35mm f/2
When did development of this lens that was faster (brighter) than the Nikkor-S Auto 35mm f/2.8 released in 1959 begin? Design reports preserved at Nikon indicate development began around 1961 when development of 28mm, 200mm, 500mm, and other reflex lenses had successfully been accomplished. Initial trial production began around the middle of 1961, but mass production was abandoned due to criticism that the lens was too large. A year later, trial production of a second, smaller version of the lens began, but that lens was also scrapped due to unsatisfactory performance. As I have said before, there was a strong desire to standardize the filter/attachment size for lenses for the Nikon F at 52 mm, and developers may have believed that even a fast lens should not be particularly large with the W-Nikkor 3.5cm f/1.8 for the Nikon S mount that Sato introduced in Tale 3 in mind. After further consideration and some revisions, the third version of the lens finally reached the mass production stage. The Nikkor Auto 35mm f/2 was released in 1965, four years after the first prototype was developed.
AI Nikkor 35mm f/2S predecessors
The Nikkor Auto 35mm f/2 gained popularity as a fast wide-angle lens. Although its title of the fastest 35mm lens for the Nikon F was lost to the Nikkor Auto 35mm f/1.4 released in 1971, it was updated with lens elements to which a multi-layer coating had been applied and released as the Nikkor Auto 35mm f/2 C in 1973. In 1975, the lens was reborn as the new Nikkor Auto 35mm f/2 with a completely new exterior. In 1977, the Automatic Maximum Aperture Indexing (AI) system was adopted to create the AI Nikkor 35mm f/2, and in 1981, the AI Nikkor 35mm f/2S was released with changes to focus operation and design near the aperture ring. Despite repeated updates and improvements over the course of five generations, the basic optical structure remained unchanged, and the lenses were used and appreciated by a great many photographers. That's not all. This long seller continued to be sold even after the release of its direct descendant, the AI AF Nikkor 35mm f/2S developed for the new age of AF cameras.
The AI Nikkor 35mm f/2S was designed by someone whose name is frequently mentioned in these tales, Yoshiyuki Shimizu. Shimizu began working at Nippon Kogaku K. K. (now Nikon Corporation) in 1952. He studied lens processing and mechanics, as well as optics at Nikon's technical training center. After engaging in lens polishing for a time, he was transferred to the department that designed optics for camera lenses and microscopes.
Fig. 1 shows how this lens is constructed. While there were some changes to edge shapes since the Nikkor Auto, the basic shape of this eight-elements-in-six-groups lens was unchanged.
I think you can see that the structure of this lens differs from that of the Nikkor-H Auto 28mm f/3.5 (introduced in Tale 12), designed with what can be considered a standard retro-focus structure, in that the convex lens element in front of the aperture is a convex-concave doublet (compound lens) and the concave element behind the aperture is a concave-convex doublet. In addition, the third and fourth lens elements are thicker. I've covered the effects of this thick lens in front of the aperture in several past tales, but it contributes to a smaller front lens element that helps to keep the filter/attachment size at 52 mm. The doublets in front of and behind the aperture alleviate and correct high-order aberrations on the exit surfaces of the convex and concave lens elements. If we look at the cross section in Fig. 1, we can see that the concave surfaces of the lens elements in front of and behind the aperture both face the aperture. This arrangement allows light rays to naturally pass through the lens elements and is standard with lens design. However, the third biconvex and fifth biconcave lens elements work in contrast to the others. As a result, unexpected high-order aberrations occur on the rear surfaces of them. However, using doublets for these elements makes for an overall meniscus shape that suppresses high-order aberrations that occur as the speed of the lens (brightness) is increased.
Originally specializing in lens polishing, Shimizu always kept lens processing in the back of his mind when designing lenses. Such consideration can be seen in the lens cross section that is both functional and attractive.
As always, let's take a look at the rendering characteristics of this lens with actual images. The sample images for this tale were captured using the full-frame Z 6 mirrorless camera with the FTZ mount adapter. When an AI-S manual-focus lens is used with the FTZ, shooting is performed with actual aperture metering. However, as lens information is not transmitted to the camera, focal length and maximum aperture information must be registered with the camera. Registering this information enables recording of focal length information in image Exif data as well as proper operation of the camera's on-board vibration reduction function. However, information is not transmitted to the camera when the aperture ring is rotated, so users must be aware that aperture information in Exif data will not change.
Sample 1 is a distant landscape captured at maximum aperture, while Sample 2 is a photo of the same scene shot with the aperture stopped down to f/5.6. Sample 1 captured at maximum aperture exhibits flare that is slight at the center of the frame but gradually increases as the distance from the center increases. However, rendering at maximum aperture is sharp and exhibits good contrast for a fast f/2 lens of its time. In addition, the effects of lateral chromatic aberration can be seen in the form of color fringing at the edges of white buildings from the mid ranges to the edges of the frame, but the degree of lateral chromatic aberration is quite low for a wide-angle lens and can easily be corrected using lateral chromatic aberration compensation functions built into the camera or NX Studio. At first glance, imaging performance appears to be good at maximum aperture but something looks a little off from the center of the frame and through the mid ranges to the extreme edges when the image is enlarged (see the enlargements below). The image is sharp and fine details are clearly rendered at the center and edges of the frame, but sharpness and clarity seem to suffer in the mid ranges. It is almost as if the viewer's vision fails in these portions. This is the result of a form of aberration known as astigmatism. The effects are most obvious in the billboards included in the image. In this sample, resolution in the meridional direction (a diagonal line from the upper left to the lower right in the enlarged images below) is good while resolution in the sagittal direction (a diagonal line from the upper right to the lower left) is not. This makes the text on the billboards difficult to read. Whether resolution is better in the meridional or sagittal direction depends on focus position, so in some cases, resolution may be better in the sagittal direction. At maximum aperture, this lens exhibits good performance with little distortion. Its only drawback is the effects of astigmatism in frame mid ranges.
The aperture was stopped down to f/5.6 for Sample 2. The effects of astigmatism become less noticeable as the aperture is stopped down, and stopping down the aperture even further from f/5.6 to f/8 almost eliminates these effects. Naturally, stopping down the aperture also eliminates the overall sense of flare for sharp rendering throughout the entire frame.
The images below are enlargements of Samples 1 and 2, as well as of an image captured with the AI Nikkor 35mm f/1.4 S introduced in Tale 27 with the aperture stopped down to f/2. If we compare Sample 1 enlargements with 35mm f/1.4 enlargements, we can see that even at the same aperture setting, the Sample 1 enlargements exhibit superior contrast. In addition, if you look at frame mid ranges in the enlarged images, you will see that the 35mm f/1.4 enlargement seems to exhibit more flare, but that the text on billboards is well rendered and the effects of astigmatism are less noticeable. Sample 2 enlargements show an increase in overall contrast and less noticeable astigmatism than Sample 1 enlargements.
Sample 3 is roughly the same scene captured with Samples 1 and 2 at night. Sample 3 was captured at maximum aperture and Sample 4 with the aperture stopped down to f/4. In Sample 3, the light sources at the center of the frame are a little blurry due to spherical and chromatic aberrations. This makes it clear that the flare in the center of the frame with Sample 1 was caused by spherical aberration and the bleeding of blues. Sagittal coma flare and point-image distortion can be seen in frame mid ranges, and astigmatism distorts the shape of point images. Sagittal coma flare becomes more pronounced at the edges of the frame, but darker point images that do not exhibit flare indicates that the distortion of point images as a result of astigmatism has been eliminated.
In Sample 4, the flare at the center of the frame is completely eliminated by stopping the aperture down to f/4. The sagittal coma flare in mid ranges is almost completely eliminated, but point images are still distorted. It seems we need to stop down the aperture a little more. There is also a small amount of sagittal coma flare at the extreme edges of the frame. Eliminating it requires stopping down the aperture to f/5.6 as with Sample 2. The peripheral illumination falloff (vignetting) that was so conspicuous in Samples 1 and 3 is barely noticeable when the aperture is stopped down to f/4 as with Sample 4.
Just as with Samples 1 and 2, the images below are enlargements of Samples 3 and 4, as well as of an image captured with the AI Nikkor 35mm f/1.4 S introduced in Tale 27 with the aperture stopped down to f/2. If we compare the Sample 3 enlargement with the 35mm f/1.4 enlargement, we can see that even at the same aperture setting, Sample 3 exhibits less flare throughout the entire frame. In addition, if you look at frame mid ranges in the enlarged images, you will see that the 35mm f/1.4 enlargement seems to exhibit more flare, but that point images are almost perfectly circular in shape indicating very little astigmatism. Sample 4 enlargements show an increase in overall contrast and significant reduction of sagittal coma flare and astigmatism.
Sample 5 is a shot of a hydrangea captured with the aperture stopped down to f/4. The shooting distance for this image was approximately 60 cm. Background bokeh in images captured with wide-angle lenses is often said to be unattractive. One of the reasons for this is that the edges of bokeh are enhanced by sagittal coma flare. I stopped the aperture down to f/4 to prevent this. Bokeh edges are rather polygonal in shape, but the bokeh itself is relatively gentle. If you look closely, you can see how astigmatism causes bokeh to appear to flow in slightly out-of-focus portions, revealing a unique characteristic of this lens.
Sample 6 is a photo of a different variety of hydrangea captured at maximum aperture. The shooting distance for this image was approximately 80 cm. As the AI Nikkor 35mm f/2S inherited its optics from the Nikkor Auto 35mm f/2 released in 1965, it is not equipped with the close-range correction mechanism. However, the image plane remains relatively flat at distances up to around 80 cm. This image was captured at maximum aperture, so the background tends to exhibit double-line bokeh as explained with Sample 5. However, this double-line bokeh is not so noticeable in portions that are heavily blurred like the people in the background. This lens offers gentle, yet impressive bokeh characteristics.
Sample 7 is a photo of flowers (Nigella) captured at nearly the minimum focus distance and maximum aperture. As noted with Sample 6, this lens is not equipped with the close-range correction mechanism. This seems like a disadvantage as the AI Nikkor 35mm f/1.4 S introduced in Tale 27 is. However, as was explained with Sample 6, this lens exhibits rather good performance at distances commonly used with normal shooting, so the user is not likely to miss the close-range correction system with actual photos. In addition, users need not be concerned with image deterioration at the edges of the frame when shooting at closer distances if the primary subject is positioned at the center of the frame as it is in Sample 7. Instead, the lack of a close-range correction mechanism produces gentle and attractive bokeh.
But some may wonder why this is, so let me explain a little about it. The close-range correction mechanism corrects curvature of field and astigmatism with close-up shooting, characteristics that are quite common with wide-angle retro-focus lenses. Lenses equipped with this mechanism achieve pleasing imaging performance when shooting flat subjects at close distances. The performance of lenses designed for close-up shooting is balanced with a priority on imaging performance at close distances. Therefore, curvature of field and astigmatism occur for objects positioned at infinity (those blurred in the background). As a result, bokeh is distorted and may appear to radiate outward. On the other hand, lenses with which focus is adjusted by moving all elements in a lens group maintain a flat image plane for subjects positioned at infinity (those blurred in the background), achieving gentle and natural bokeh characteristics.
35mm f/2 successors
When the Nikkor Auto 35mm f/2 was initially released, the astigmatism in frame mid ranges that was pointed out in this tale was not really a problem within Nikon or without. As the only fast 35mm lens available at the time, it must have been easy to overlook this shortcoming, which was only noticeable when photos of flat surfaces captured with fine-grain film were enlarged. Nonetheless, optical designers were aware of this drawback and tried to improve performance by updating the optics with each new version of the lens from the Nikkor Auto to the AI Nikkor to the AI-S Nikkor. However, while prototypes were produced and evaluated, they never made it to mass production. Judging from data that remains, one of the reasons seems to be that (sagittal) coma flare was even worse than it was with the existing 35mm f/2. While the lens has its shortcomings, it seems it was still appealing with its excellent contrast even at maximum aperture. In addition, constraints on manufacturing costs must have made it difficult to improve on the design of this lens that was sandwiched between the faster f/1.4 lens and the smaller and lower priced f/2.8 lens.
This lens was the first 35mm lens I ever purchased. At the time, I bought it to photograph night skies so thought an f/2 lens with less coma flare at maximum aperture would be preferable to an f/1.4 lens. At first glance, the AI Nikkor 35mm f/2S does not seem to offer much of a unique character as the AI Nikkor 35mm f/1.4 S, with which the amount of flare changes dramatically with changes to the aperture setting, but it is an attractive lens for gentle rendering at short and medium shooting distances. It also offers sharp rendering from maximum aperture despite being an old lens.
* The aperture value indicated in Exif data differs because the FTZ does not transmit aperture information for manual lenses. The aperture values indicated above are correct.
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.