NIKKOR - The Thousand and One Nights No.28
Ai AF Nikkor 28mm F1.4D
Tonight's tale is a sequel to Tale Sixteen about the Noct NIKKOR. Tonight, I would like to talk about the wide-angle lens Ai AF Nikkor 28mm f/1.4D, which is similar to the Ai 35mm f/1.4 lens I talked about last night in terms of focal length, but different in imaging characteristics.
by Kouichi Ohshita
I. A wide-angle version of 35mm f/1.4
It wasn't immediately after completing the development of the 35mm f/1.4 as discussed in Tale Twenty Seven when the development of the 28mm f/1.4 was launched. This was because the development of the wider-angle, large-diameter f/1.4 lens was expected to involve great difficulty based on the experience we had in developing the 35mm f/1.4 and 28mm f/2 lenses. It meant that the aberration of the sagittal coma flare, with which you may not be familiar, would become more difficult to correct. As I mentioned in Tale Sixteen about Noct Nikkor, the sagittal coma flare is an aberration, occurring when a point light source located on the periphery of an image frame makes an image like a bird spreading its wings when a night view or an astrophotograph is taken with a large-diameter or wide-angle lens set at full-open aperture. This flare occurs more significantly with larger diameter lenses and wider angle lenses.
It was known that effective correction of sagittal coma flare would require the use of aspherical lens, though the volume production technique of highly accurate aspherical lens was not yet fully established at that time. The first successful volume production of aspherical lenses was the OP fisheye introduced in 1968, though the accuracy of the aspherical surface had to be further refined for application to large-diameter lenses.
A turning point was the Noct Nikkor launched in 1977. The manufacturing method for aspherical surface was based on polishing, which nowadays would be considered unsuitable for volume production. However, the successful volume production of that lens would have been a very wonderful, epoch-making event for optical designers and aspherical lens engineers. "Well, let's create a wide and large-diameter lens next time!" The desire to develop an unprecedented lens grew among designers and aspherical surface production engineers. In this way, the development of the 28mm f/1.4 got off the ground.
II. Inherited lens design
Development of the lens advanced along with optical design and selection of manufacturing method of aspherical surfaces. While the optical design team was exploring the optimum lens type and profile of the aspherical surface, the aspherical surface production team was seeking to identify a novel processing method best suited for the aspherical lens designed. Then, taking into account the information from the aspherical surface processing team, lens design was pushed forward.
The design of the 28mm f/1.4 lens was completed, and at the same time, a processing technique which allows grinding of precision aspherical surface was developed, achieving a quantum leap in accuracy and productivity. This technique has been applied also to AF20-35/2.8 in addition to this lens. After several years from the start of development, a working trial lens of 28mm f/1.4 took shape.
However, the quality assurance department stopped volume production of the prototype lens. The reason was that the performance was not as high as anticipated. According to the in-house evaluation report, the performance was similar to that of the 28mm f/2, but inferior in the shorter focusing ranges. In the past as in the present, the designers and engineers cannot deny the results of the evaluation by the quality assurance department. The quality of NIKKOR lenses has always been guaranteed by the stringent judgment of the quality assurance.
The design of the lens went back to a starting point, and was entrusted to younger colleagues. Within Nikon, as a rule one optical designer is responsible for one product from start of design through volume production, though this lens involved a total of four optical designers and it was at last completed after a lot of hard work. In 1994, the lens was placed on the market about ten years after the start of development.
III. Lens performance
The figure shows the configuration of the 28mm f/1.4 lens. This lens system is characterized by a double-convex element with a large curvature arranged immediately before the diaphragm, and an aspherical lens with a high aspherical degree as you can see at first glance.
However, I hope you will forgive me for going back to a somewhat difficult-to-understand subject such as aberration. Here, in the case of a wide-angle lens in which a many concave elements are arranged in the front group, it is difficult to correct the sagittal coma flare which is believed to occur at a concave surface with a large curvature. Therefore, in this lens we are now discussing, the curvature of individual concave elements is reduced to the smallest possible extent, and a convex lens with a large curvature is combined with an aspherical lens to successfully correct a possible sagittal coma flare.
Another feature of this lens includes a complicated focusing system, which evolved from the floating adjustment adopted in the 24mm f/2.8. The complicated focusing system helps correct adequately not only astigmatism in the shorter focusing ranges, but also spherical aberration and coma aberration. Optical design is discussed in terms of "a degree of freedom." It means that any one change in an interval between lens or change in curvature could correct one aberration independently. Focusing requires the change of one interval between lens, and the change of a total of four intervals between lens for additional correction of spherical aberration, coma aberration and astigmatism. In this way, the problems in the performance of the working trial lens were resolved and a high-performance lens was realized.
IV. Imaging characteristics
Finally, let's check what this lens can do based on the sample photographs. Sample 1 is an astrophotograph of the Leonid meteor shower in 2001, which was much talked about in newspapers and on TV. For ordinary stars, a dark star invisible to the naked eye could be captured by mounting a camera on an apparatus known as an equatorial telescope, which tracks the diurnal motion of the star, and accumulating the light of the star by using a long exposure. However, for a meteoric swarm, the duration of the bright light is limited to a very short period of time, therefore, a combination of fast film and a large-diameter lens is essential. In addition, since it is not known when and in which direction the meteoric swarm will stream, a lens with a wider angle of view are more advantageous to capture the meteoric swarm in the frame. This lens, equipped with both a wide viewing angle of 28mm and fast f/1.4, may be just the right lens to take a shot of a meteoric swarm. Sample 1 was taken at full-open aperture. A pale blaze of light due to sagittal coma flare is slightly seen in the surrounding bright stars, though you may notice it is not very conspicuous. In practice, however, astrophotography at full-open aperture could result in noticeable vignetting in some cases. Therefore, one or two stop-down setting provides better results in the photography of subjects other than meteoric swarm. This Sample Photo was obtained by reading the negative film through a scanner, increasing the contrast and compensating the brightness of the edge of the image field.
Sample 2 shows a night view. You may notice that the sagittal coma flare, visible in the astrophotograph involving the point light sources of the stars, is less conspicuous in this Sample Photo. This lens is easy to use, providing pictures with a visual sharpness both at full-open aperture and stop-down aperture.
By my guess, this lens was developed to offer a wide-angle version of Noct Nikkor. Precisely for this reason, the first working trial lens might be rejected. There might have been an acceptable solution with a performance that would be similar to that of the well-established popular 28mm f/2 at the one-stop brighter, full-open aperture of f/1.4. However, the engineers involved in the development project at that time would have not been able to accept that solution.
They were pursuing a lens system to offer unprecedented performance like Noct Nikkor as they took so much effort to incorporate the highly precise aspherical lens.
The focal length of 28mm is typical of standard wide angle, and it corresponds to an angle of view of 42mm when used with D70S, D2X or other digital cameras and it is also easy to use as a large-diameter standard lens for digital SLR cameras. This is one of the lenses I usually use, and the opportunity to use this lens is growing so much more in the digital age.