NIKKOR - The Thousand and One Nights No.41
Ai AF Nikkor 85mm F1.4D (IF)
In this forty-first tale I'd like to talk about the AF Nikkor 85mm f/1.4D IF, which is favored by many for portrait photography. As a standard lens for portrait work, this lens has proven just as popular in the current age of digital photography as it was in the film era. What are the features that give this lens its appeal, and what anecdotes are there from the period when it was being developed? Tonight we'll uncover the secrets of this standard lens.
by Haruo Sato
I. Development history
We'll now reveal the story of the development history of the AF Nikkor 85mm f/1.4D IF. The man who coordinated the optical design of this lens will be familiar to readers of Nikkor's Tales of the Thousand and One Nights-–he is none other than Koichi Oshita. Mr. Oshita, who was then employed in the First Optical Department of the Optical Division, began work on the design of the 85mm f/1.4 when he took over from colleague Mr. Yanagisawa. Design work commenced in the early summer of 1992 and (including an investigation of how the lens rendered images) was completed close to the end of spring 1993. Mr. Oshita did not simply set out to design a large-aperture lens-–he devoted himself wholeheartedly to developing a large-aperture medium telephoto lens with the ideal rendering characteristics for portrait photography.
Investigating the bokeh (blur quality) and rendering characteristics of successive generations of great lenses, discovering the optimal aberration balance, and deliberating on how best to match the lens with the AF system, he created a new type of lens and a new focusing method. Adopting the inner focus method, he discovered a scheme for obtaining good bokeh while still maintaining the imaging section's sharpness. What he came up with was the AF Nikkor 85mm f/1.4D IF. Since this lens used a large amount of expensive glass, it cost more to make than had been envisaged. Despite this factor, however, the launch price was low, making this a bargain Nikkor lens. Trial production began in spring 1994 and mass production began in the winter of 1995. At long last, in December 1995, the lens was released. There is no doubt that this was a labor of love.
II. The structure and characteristics of the lens
Let's first take a look at a cross-sectional diagram of the lens. At first glance the lens appears to be a variant of the Gauss-type lens. However, its structure deviates significantly from that of the Gauss-type lens. The most notable characteristic of this lens is the action of the triple convex group structure. The front group, which is fixed during focusing, plays the role of a convex converter; the middle group, which plays the role of the master lens-–including aperture stop-–performs the focusing (that is, inner focusing is employed); and the rear group, which is also fixed during focusing, is a compensating group that prevents aberration fluctuations at close range. Fixing the rear group is the touch of a master craftsman specifically intended to curb fluctuations at close range due to upper coma aberration and field curvature, and also serving to achieve higher performance. During the development of this new type of lens devised by Mr. Oshita, application was made for a patent in Japan, and several years later a patent was obtained in the United States. Thus, Mr. Oshita's invention of this unique lens was acknowledged globally.
III. Imaging characteristics and lens capabilities
Let's take a look at the lens' aberrational characteristics, with reference to the design records. First, the most notable characteristics are that the spherical aberration is small and that the chromatic spherical aberration is well balanced. These are even more advantageous in the current age of digital photography. Other major characteristics are the slight persistence of coma aberration and the way in which the persistence of the lower coma aberration in particular makes for attractive bokeh. Also worthy of note is the low level of distortion, which is within -0.6% for the entire image. Additional major characteristics are the low level of close-range aberration fluctuation and the deft correction for field curvature, astigmatism and chromatic aberration of magnification.
Let's take a look at the spot diagram to find out how well the lens performs point imaging. Although the points extend outwards from the center to the periphery, with a faint flare around the circumference, there is good concentration of points around the center and high resolution can be expected. Also, if the focus is represented diagrammatically, the point image does not feature sharp outline edges, indicating that comparatively good bokeh will be generated.
What kind of pictures, then, does the AF Nikkor 85mm f/1.4D IF take? Let's consider this question based on the photographic results from both long-distance action photography and portrait photography.
With the aperture opened to f/1.4, high resolution is achieved, with a particular sense of resolution close to the center of the image, although there is a faint peripheral flaring. The flare gradually increases-–albeit slightly-–from the center of the image to the periphery, and the sense of resolution diminishes somewhat. Although there is a slight lack of light intensity at the four corners due to the effect of vignetting, the level is satisfactory for this class of lens. The background bokeh is weakest at the center of the image and tends to become gradually more pronounced toward the edges of the image, which are also subject to the effect of vignetting. Overall, however, the lens produces relatively good bokeh. If the lens is stopped down to f/2, the faint surrounding flare disappears, resolution is high, and the contrast improves, except at the four corners. The bokeh and vignetting are also improved and edges become smoother-–hence the more pleasing bokeh. When the lens is stopped down to f/2.8-f/4, the sense of sharpness increases further and proper contrast is obtained right up to the periphery of the image. When the lens is stopped down to f/5.6-f/11, there is improved definition, which extends to the periphery of the image, and the entire image exhibits a uniformly high picture quality. The optimal contrast enables images to be rendered that are rich in gradation. When the lens is stopped down to f/16, the point imaging is uniform in shape; however, there is a slight diffraction effect and sharpness is slightly reduced. Given the relationship between the bokeh, blur quantity and depth of focus, an F-stop setting of close to f/2 has been regarded as best for portrait photography. F-stop settings in the region of f/5.6-f/8 have been regarded as suitable for shooting objects or photographing landscapes.
Let's take a look at the lens' imaging characteristics by examining some example photographs. The first example photograph was shot with the aperture opened to f/1.4. It was shot with vegetation in the form of flowers in the foreground and background-–the most testing conditions for the assessment of bokeh. The sharpness in the region of focus can be appreciated by looking at the texture of the hair, the eyelashes, and the area around the eyes. Turning to the regions in which there is gradual defocusing, it can be seen that there is smooth blurring in both the foreground and background, but that the occurrence of cross-eyed blur has been prevented. Although the softness diminishes slightly at the very edges of the image, due to the effect of vignetting, there is a richness of gradation, and-–for this class of lens-–natural images with a pleasing bokeh are obtained.
The second example was shot with the lens stopped down to f/2.2. Stopping down has improved the image sharpness and enlarged the field of focus somewhat. Also noteworthy is the more pleasing bokeh. The outlines of the bokeh are smoother and the vignetting effect at the very edges of the image has been reduced. Optimal contrast has also been achieved and a natural image that is rich in gradation has been obtained.
The personality of Koichi Oshita
Koichi Oshita and I joined Nikon at the same time, and from our pre-assignment induction training onwards, we spent time together. We would also take pictures, visit camera shops and go on trips together. From that moment on, the two of us engaged in an exhaustive comparison of the characteristics of lenses of every description and from every manufacturer, through which we came to understand the characteristics of the photographic lens. These experiences laid the groundwork for our work in writing the tales of NIKKOR the Thousand and One Nights. Mr. Oshita had always had a predisposition for approaching matters in an analytical fashion. More than anyone else, he was totally scrupulous in his inclination to submit the data to close examination. In my younger days we would often go for a drink together after work. On these occasions we would almost invariably end up in a heated discussion on the topic of photographic lenses. Although many years have passed since those days, I would like to relate an anecdote that typifies the man-–one of numerous such incidents.
One day we went drinking with some colleagues from work and ordered sake. The requisite drinking vessels, a glass inside a masu (a wooden square measuring cup), were set out for us. The barman filled them to the brim with sake-–the sake overflowing from the glass to fill the masu as well. "Some people are stupid enough to spill sake from the masu when they drink it from the glass!" Mr. Oshita exclaimed triumphantly. "That's right!" we all replied, taking our glasses out of the masu so as to drink from them. Before long, we heard a groan. When we looked up, our eyes were met by the sight of Mr. Oshita, his chest soaking wet. We all laughed heartily-–he'd personally proved the point himself! This was the same Mr. Oshita who devoted himself to his work with total passion, coming up with fine blur quality as well as new lenses, and creating standout Nikkor products. On the strength of this record of achievement, he is currently in charge of the design and development of the lens sections of digital cameras and will be developing more fine products for Nikon in the future.