3. Imaging characteristics and lens capabilities
- Cross-sectional diagram
Let's first take a look at a cross-sectional diagram of the lens. The lens is a typical Tele-Sonnar lens and boasts a simple and logical three-group, four-element structure, with a convex lens on the left, cemented convex and concave lenses in the middle, and a convex lens on the right. With its asymmetrical aperture, it is classed as a telephoto lens. Accordingly, its overall length is short compared to its focal length. These Tele-Sonnar lenses, with their thick walls, are characteristically long and thin, with a small front-lens assembly and a small filter diameter designed to keep the incident light rays closer to the optical axis. In technical terms, the entrance pupil can be disposed closer to the object. In terms of aberration, the lens is prone to pin-cushion distortion on account of its asymmetry and does not do such a good job of correcting chromatic aberration of magnification. Although the lens exhibits excellent spherical aberration correction, the difficulty inherent in controlling the variations in spherical aberration corresponding to different colors shows itself in the lens' tendency to over-correct spherical aberration at short wavelengths (from blue to bluish purple). This is due to the simplicity of the lens's structure. The degree to which the lens can control this type of aberration is dependent on the designer.
What kind of pictures does the NIKKOR-QC 13.5cm f/4 take? Let's consider this question based on both the lens's aberration characteristics and the photographic results.
First, let's read the design report. As regards aberration correction, the main features of this lens relate to spherical aberration and field curvature. The lens under corrects for spherical aberration, resulting in a pleasing background blur. In addition, since the field curvature is relatively large, the lens under corrects both the sagittal image (S image) and the meridional image (M image). In particular, the M image is greatly under-corrected, resulting in a considerable degree of astigmatism. In addition, under-correcting the S-image surface serves to suppress sagittal coma flare. Although this technique sacrifices the flatness of the image plane slightly, it avoids the "blurry" images caused by flare.
The images formed by point sources of light are shown in a spot diagram. The focus of the point images is good for the center of the lens, indicating that sharp images will be formed. However, as the image height increases, there is a gradual tendency toward front focus, due to the low field curvature. There is little flare; however, resolution drops off slightly toward the periphery of the image. The meridional coma flare is slightly greater than the sagittal coma flare.
In terms of overall imaging characteristics, the center of the image produced by the lens is sharp and of high resolution, while there is a tendency for the resolution to drop off toward the periphery of the image, due to front focus resulting from the field curvature. As the point images do not undergo any unnatural transformation, the lens seems to produce images that are unaffected and devoid of any quirks. Notably, the low field curvature produces a pleasing blur (bokeh) quality in the image background. A nice and simple blur quality can be expected, coupled with a tendency for the bokeh to increase closer to the edge of the image. The lens is also good at correcting for distortion (which is supposedly difficult to correct for), with the level of distortion being around 0.6%.
Next, let's take a look at the photographic results of this lens. At f/4 (full aperture), the central area of the image exhibits high resolution and relatively good contrast. From the center outward toward the edges, the image progressively softens and the resolution declines slightly, in line with the tendency toward front focus. However, the lens produces soft fault-free images without any unsightly rivers running through them, and offers apparently higher resolution than is suggested by the design values. Stopping down to 5.6 improves the sharpness of the central area of the image and increases the area of the image that appears sharp. The periphery of the image is also improved, becoming sharp enough to qualify as part of the high-quality region of the image. Stopping down to f/8-f/11 extends the high resolution area to the periphery of the image and produces a consistently high image quality throughout the entire picture. The contrast level is also just right, with images that are rich in tonal gradation, as opposed to exhibiting a stark two-tone contrast. When the lens is stopped down to f/16, the shape of the point images becomes uniform; however, the overall sharpness is reduced due to the effect of diffraction. For the sharpest results, it is probably best to stop the lens down to f/8 or f/11, while the f/4 aperture setting is probably best for shooting portraits.
Let's take a look at the lens' imaging characteristics by examining some example photographs. The first example is a portrait. As demonstrated by the texture of the hair and eyelashes, the resolution and contrast are just right, yielding a natural-looking image that is rich in gradation. Also noteworthy is the splendid bokeh of the background. The photograph has a melting-like blur effect, which is probably another reason why Mr. Duncan favored this lens.
The second example photograph is a backlit snapshot. It was shot with the lens stopped down, producing a quirk-free image with a consistent sharpness that extends right to the edges of the picture. It is worth noting that the image does not exhibit the aforementioned "two-tone" high contrast level. It is clear that although the image was backlit by a flood of light from the clear sky, even the dark areas of the image have been well reproduced and the level of contrast compression is appropriate.
- Nikon SP NIKKOR QC 13.5cm f/4 Aperture: f/2.8, shutter speed: 1/60 sec Film: Tri-X Film development: microfine 1:1 Enlargement development: Korectol Enlargement lens: EL NIKKOR 50mm f/2.8 (at f/11) Photographic paper: Fuji Film Varigrade (equivalent to grade 1.5-2 paper)
Shot in August 2009
- Nikon SP NIKKOR QC 13.5cm f/4, Aperture: f/11, shutter speed: 1/250 sec, Film: Tri-X Film development: microfine 1:1, Enlargement development: Korectol, Enlargement lens: EL NIKKOR 50mm f/2.8 (at f/11), Photographic paper: Fuji Film Varigrade (equivalent to grade 2 paper)
Shot in September 2009