Secrets of a popular standard lens Tale 37 : W-Nikkor 3.5 cm f/2.5

Firstly, I'd like to talk about how the development of the Nikkor lens 3.5 cm f/2.5 progressed. For the design of the optical system, the company relied on Hideo Azuma, whom I mentioned in Tales Three and Twenty-Nine. Mr. Azuma is known as having designed a number of wide-angle lenses in a short space of time. When I read the optical design report for the 3.5 cm f/2.5 lens, I was surprised to find that this lens was originally designed with f/2.7. This means that the lens was developed based on the following design approach: the power arrangement of the lens is first determined based on the paraxial theory, with consideration given to minimizing the possibility of third-order aberration. The aperture is then increased gradually. If the design is poor, this approach will fail when apertures are increased. It was found that the original design caused less residual aberration, and aberrations of higher orders were rare. This approach may have been adopted because more appropriate tools were not available in those days. Mr. Azuma, who decided to use such a design approach, must have had a thorough knowledge of Gauss type lenses.

Mass production of the 3.5 cm f/2.5 lens began in 1951. The lens came onto the market in July of the same year and sales continued for nine full years. The lens barrel was originally made of brass and thus the lens was heavy. Subsequently, however, a light alloy was introduced to provide a lens barrel similar to that of the 3.5 cm f/1.8 lens, thus making the lens an easy-to-use popular standard. In addition, as Mr. Oshita explained in Tale Eight, the Nikkor lens 3.5 cm f/2.5 for the Nikon S series cameras enjoyed an established reputation and was easy to manufacture; subsequently, design modifications led to changes in radius of curvature and the type of glass used, with the remodeled lens becoming the standard lens for NIKONOS cameras. This proves that Mr. Azuma's basic design was exceptional. Lenses with a well-defined basic design can be used for many purposes.

Take a look at the cross-sectional view. This lens is a typical Gauss type lens. It has six elements in four groups composed, from the left, of convex lens, compound convex/concave cemented lens, compound concave/convex cemented lens, and convex. Between the inner two compound cemented elements, an aperture is provided that provides symmetrical configuration. Gauss type lenses feature easy control of spherical and chromatic aberrations and also provide compensation for lateral chromatic aberration and distortion whether one wants it or not, due to the symmetrical configuration. Therefore, they are easier to manufacture with larger diameters and respond to wider picture angles compared with Triplet type and Tessar type lenses. In fact, for a Gauss type lens to assure satisfactory performance, a picture angle of 60 to 70 degrees is considered the limit. For lenses with picture angles wider than the limit, Orthometar type and Topogon type lenses discussed in Tale Twenty Nine are superior. On the other hand, Gauss type lenses have the disadvantage of poor compensation for saggital coma flare. How this problem can be overcome is a challenge for optical designers.

Now, let's see what the W-Nikkor 3.5 cm f/2.5 can do based on both aberration characteristics and actual images.

First, let's look at the optical design report. This lens focuses the characteristics of aberration correction on spherical aberration and curvature of field. Spherical aberration is kept undercorrected. This helps improve background bokeh. In addition, the field curvature is relatively large, resulting in undercorrected and expanded S (saggital) and M (meridional) image surfaces. However, astigmatism is reduced except at the periphery, providing well-compensated images for both M and S. The undercorrected S image may be intended to suppress saggital coma flare. Mr. Azuma must have suppressed the kite-shaped flare while slightly compromising image flatness. A spot diagram indicates the condition of image formation of a point source of light. In the center, object points are found well arranged and thus sharp image formation is expected. However, as the image height increases, the image is increasingly susceptible to front focus due to the undercorrected curvature of the field. As for coma flare, meridional coma flare is slightly more likely to occur, while kite-shaped flare specific to saggital coma does not occur.

The imaging characteristics of this lens may be summarized as follows: Sharp image formation is attained in the center with moderate resolution; on the periphery, imaging tends to involve front focus caused by the effects of field curvature with gradually degrading sharpness due to the occurrence of flare. However, object points never distort unnaturally, thus contributing to straightforward imaging. In addition, distortion is limited to 1 percent and below, and lateral chromatic aberration is also reduced; therefore, improved sharpness is achieved by stopping down the lens a little.

Next, I'll look at actual shots. At the full-open aperture of f/2.5 to f/2.8, moderate resolution is seen in the vicinity of the center, with sensitive imaging. From the center to the periphery, images are susceptible to front focus and thus become soft. However, there is no ugly image blurring, with natural, easy imaging achieved. It is evident that light has fallen off around the four corners, though the reduction of light intensity is normal for wide-angle lenses of the time. When stopping down the lens to between f/4 and f/5.6, sharpness in the vicinity of the center is improved, leading to an expanded sharp region. The image on the periphery is also improved with increased sharpness. The entire image, except at the extreme periphery, reaches the satisfactory image region. Insufficient light intensity at the periphery is also eliminated. When closing the aperture to between f/8 and f/11, improved resolution is attained even on the periphery, and satisfactory image quality is obtained uniformly over the entire image. Contrast also becomes well balanced and the lens offers wide-range tone reproduction without overly sharp, tight contrast. When stopping down to between f/16 and f/22, object points become geometrically uniform, though the overall image suffers from a fall in sharpness due to the effects of light diffraction. For better sharpness, it is effective to stop the lens down to between f/8 and f/11, while for soft imaging suited for portraits, it is recommended to select apertures f/2.8 to f/4.

Next, let's check the imaging characteristics based on sample photos. Sample 1 shows a portrait. As can be seen from the texture of the hair and clothing, natural imaging is attained with sensitive lines, moderate contrast and rich gradation. In addition, as the patterns on the wall show, the image contains no unnatural distortion or flare.

Sample 2 is a scenic snapshot. It was taken by stopping down the lens, and sharpness is found even at the periphery, resulting in straightforward imaging. Worth noting is the fact that there is no significantly high contrast. The photo was shot in strong sunlight under a clear sky, although the shaded area is well reproduced, which indicates that contrast is compressed to an appropriate degree.