Overcome Variation in Close-Range Aberrations
As I told in Tale Twelve, in the course of the development of 2.8cm f/3.5 (released in March of 1960), we have found a new lens type to overcome barrel type distortion and a deterioration of optical performance on the periphery of the frame, which had been typical weak points of a retrofocus type lens, but it was not the story that a wide-angle lens of 24mm had been developed without difficulty. To tell the truth, a retrofocus type lens having a shorter focal length than 28mm was completely in uncharted territory and the lens power arrangement for obtaining high optical performance was in the stage of try and error. When I took a look at the optical design report, a large number of design data of those days kept in the report made me imagine how eagerly they had concentrated on development of the lens.
As you know, it was an era when a high-speed computer of today did not exist. It must have been a time consuming labor to let light rays through all optical surfaces over entire wide angle of view to form an image. Moreover, there was another defect left to overcome in a retrofocus type lens. It was variation in close-range aberrations mentioned at the end of Tale Twelve regarding the NIKKOR-H Auto 2.8cm f/3.5.
In a retrofocus type lens, astigmatism is gradually produced as focusing at a close object and, as a result, blurred images formed on the periphery of the frame as shown in Fig. 2. The faster the lens, the wider the angle of view, and the nearer the distance to the object become, the more conspicuous the deterioration of optical performance upon focusing at a close object becomes. Consequently, optical performance on the periphery is deteriorated.
In the NIKKOR-H Auto 2.8cm f/3.5, this was taken measures by limiting the closest shooting distance by 60cm. In every 28mm lens released from competitors later than Nikon, the closest shooting distance was shorter than that of the NIKKOR-H Auto 2.8cm f/3.5, so that the request for a wide-angle lens capable of shooting closer object became urgent day by day. However, Mr. WAKIMOTO, Zenji did not agree to simply shorten the closest shooting distance. Indeed the closest shooting distance could be shorten by increasing a bit of the amount of displacement of the lens, but it could not provide excellent optical performance satisfying most users in a close shooting distance. As a result, developing super wide-angle lenses necessary to be designed in the future may have been on the rocks unless solving the problem right away. The thought like this produced an idea of a close-range correction system (or floating adjustment), which is generally applied to wide-angle lenses in these days.
Lens performance and imaging characteristics
The NIKKOR-N Auto 24mm f/2.8 released almost ten years later than the release of the NIKKOR-H Auto 2.8cm f/3.5 was a 7-group, 9-element retrofocus type lens as shown in Fig. 1. The almost ten years tells their great pains to have been taken. The lens was born by the basic lens design in the charge of Mr. SHIMIZU, Yoshiyuki in combination with Mr. WAKIMOTO's idea, the close-range correction.
In comparison with the NIKKOR-H Auto 2.8cm f/3.5 of Tale Twelve, although the boundary between the front lens group and the rear lens group is difficult to define, assuming that the front lens group having negative refractive power is up to third lens element counted from the object side and the rear lens group having positive refractive power is the lest of the lens elements, it can be thought that the lens (NIKKOR-N Auto 24mm f/2.8) is improved speed and widened the angle of view by adding a negative meniscus lens to the object side of the front lens group of the lens (NIKKOR-H Auto 2.8cm f/3.5) and by making the positive lens locating the most object side of the rear lens group and the positive lens locating the second from the image side of the rear lens group of the lens (NIKKOR-H Auto 2.8cm f/3.5) be cemented lenses.
The focusing of the close-range correction system, the most important feature of the lens, is carried out by separating the lens into two lens groups between the sixth lens element and the seventh lens element, and by narrowing the separation between the two lens groups upon focusing as indicated by arrows in Fig. 1.
Please excuse me if the following story is a little difficult. The separation between the sixth lens element and the seventh lens element is designed such that the variation in the separation makes almost no effect on spherical aberration and makes effect only on astigmatism. Accordingly, the focusing is carried out such that astigmatism produced upon focusing at close object is canceled out by astigmatism produced by varying the separation.
Although it is easy when you come to think of it, the invention of the close-range correction system made it possible to make great strides in development of a retrofocus type super wide-angle lens.
NIKKOR-N Auto 24mm f/2.8 lens,
f/8, Auto (shutter speed),
©2002/2003 Kouichi Ohshita
NIKKOR-N Auto 24mm f/2.8 lens, f/4,
Auto (shutter speed), FUJI TREBI
©2002/2003 Kouichi Ohshita
Let's take a look at how this lens performs based on the examples.
Usually, I use the AI Nikkor 24mm f/2.8 (released in 1977) regularly. At this time, I compared the NIKKOR-N Auto 24mm f/2.8 (hereinafter called NIKKOR-N Auto) with my AI Nikkor 24mm f/2.8 (hereinafter called AI Nikkor). As a result, I again realized that the NIKKOR-N Auto had almost equal high optical performance to my AI Nikkor, which has the same optical elements as the newest AF Nikkor 24mm f/2.8D.
With the aperture stop fully open f/2.8, bright objects are surrounded by faint flares but when stopping down by two steps, flares become inconspicuous.
Example 1. is shot with the aperture stopping down at f/8. You can easily notice that the image is very sharp all over the image frame.
Image flatness, my AI Nikkor is better. However, the difference is very little such that shooting the same object with f/4 or f/5.6, looking at the four corners of the image frames in detail, and, finally, you can barely notice the difference. Of course, optical performance of the NIKKOR-N Auto is high enough to feel no dissatisfaction from the shot images.
Moreover, distortion is corrected satisfactorily, so you may not feel any distortion in the portion of a building shown in Example 1.
As the NIKKOR-N Auto used this time is applied with multi-layer coating, even if I shot an image with the sun in the image frame, any conspicuous ghost image or flare was not recognized. I got the impression that color reproduction was almost equal to that of my AI Nikkor. However, you may notice a subtle difference in the tone of color when you shoot against light. This is because chromatic aberration and flare affect the tone of color. So, this is not the case limiting to the lens, you had better stop down a little upon shooting against light.
Example 2. is shot at a close distance with f/4. Since the object is three-dimensional, it is difficult to see the effect of the close-range correction system. Don't you think the defocused image without ugly image blurring on the periphery is rather gentle as a wide-angle lens ?
The NIKKOR-N Auto 24mm f/2.8 was applied with multi-layer coating later in 1972, was modified to "new Nikkor" in 1975 together with the 24mm f/2,
and was continuously produced for about 10 years until the release of the AI Nikkor 24mm f/2.8 in 1977, which had more improved optical performance, and it still enjoys wide popularity.
By the way, the AI Nikkor 24mm f/2.8S now available uses a newly developed multi-layer coating (Super Integrated Coating, or SIC) for even better color balance and clear image.
There's more, the technique for widening an angle of view and the close-range correction system were succeeded to subsequent wide-angle lenses and are continued living in respective lenses.
In the letter "0.3m / 1 feet" shown on the focusing ring, not only the information that a close object could be shot was simply indicated, but also lens designers' satisfaction and pride, which lens designers of the single-lens-reflex camera had finally overcome variation in close-range aberrations having been troubling them, were also engraved.