NIKKOR - The Thousand and One Nights No.8

W Nikkor 35mm F2.5

Not all lenses called "NIKKOR" lenses are for SLR cameras. They are valuably used in various fields, not only taking lenses for Nikon S series, M39 ("Leica screw") mount, and large format cameras, but also lenses for enlarging, photo engraving and film scanner.
In this Eighth Tale, I'd like to discuss the W Nikkor 35mm f/2.5 (amphibious- on land and underwater) interchangeable lens for all weather camera "NIKONOS" series.

by Kouichi Ohshita

I. The birth of "W Nikkor 35mm f/2.5"

"W Nikkor 35mm f/2.5", still used as an interchangeable lens for "NIKONOS V" at present, was born in 1963 at the same time as the first "NIKONOS" was released and since then it has been selling for long 37 years. Among the current Nikkor lenses for 35mm (135) format, it may be the longest-selling lens. First, I'll introduce its birth and history.

It may be a well known episode that "NIKONOS" was made on the basis of the camera "Calypso" (1961) of La Spirotechnique as it is introduced in "NIKKOR Club Quarterly" magazine and "Nikon Family Cousins" (Part 17) carried in this web page.
At the time when Nippon Kogaku K.K. made this "NIKONOS", it planned to succeed the mechanism of "Calypso" almost as it was, but regarding the lens, it decided to switch from "SOM BERTHOIT, FLOR 35mm f/3.5 ("FLOR" is the nickname of tessar type lens of BERTHOIT) to Nikkor Lens, and the first born was this "W Nikkor 35mm f/2.5."

If you think of "3.5cm f/2.5" lens for Nikon S or M39 screw mount cameras upon hearing the specifications of "35mm f/2.5", you are quite smart. The fact is that "W Nikkor 35mm f/2.5" was designed based on the optical system of "W-Nikkor 3.5cm f/2.5" that was being sold for S/M39.
The reason why this lens was to be equipped with "NIKONOS" have to be just guessed at this distance of time, and I guess that it may have been the decisive factor that the dimension of this lens proved to be fortunately just put inside the lens barrel of the lens for "Calypso", in the process of filling the requirement of "equipping a lens of higher speed and higher performance".
"W-Nikkor 3.5cm f/2.5" was released in July of 1952, two years later than the release of the "Nikon S". The optics of the lens was designed by Mr. AZUMA, Hideo (see Tale Three), same as "W-Nikkor 3.5cm f/1.8".

At the time of release, it was the fastest 35mm (3.5cm) lens and its image quality was highly evaluated but it couldn't be denied that the design of the lens was somewhat old-fashioned. Therefore, the curvature and the glass materials were reviewed based on this lens and the newly designed lens was equipped to "NIKONOS".

"W Nikkor 35mm f/2.5 (amphibious)" has come to the present after several minor improvement including the change of lens barrel design such as the depth of field indicator, the change of the color of the lens barrel and the focusing knob from silver to black, the change of the lens coating to a multi-layer coating, and so on.
The lens has the history of half a century counted from the time of the development of "W-Nikkor 3.5cm f/2.5" which served as the model of the optical system for that lens.

II. Rendition characteristics and lens performance

As shown in (Fig. 1.), "W Nikkor 35mm f/2.5" lens is consisting of a 4-group, 6-element typical Gauss type lens and a thick parallel plane glass plate arranged in front of the lens. The parallel plane glass is a protector, which serves, so to speak, as a filter in the air, but as a water-tight window under water that protects the internal lens against high water pressure.

The structural feature of Gaussian type lens is that the aperture (iris) stop is arranged in the middle of the lens and the diameter of the lens becomes small as though it is constricted at the position near the stop. It has an advantage of restricting the diameter of the stop smaller than that of a 3-group, 4-element typical Tessar type lens of the same focal length and same speed. It is due to this feature of Gaussian type that a fast f/2.5 lens could be incorporated in the mechanism of the lens barrel of the lens for "NIKONOS".

Optical feature of Gaussian type is, as mentioned in The Second Tale "AI Nikkor 50mm f/2" of this series, that spherical and chromatic aberrations are well corrected. This lens also makes the most of that feature, and owing to the well corrected chromatic aberration and the effect of multi-layer coating (the current model is applied with the multi-layer coating), the clear image with excellent color reproduction will be obtained.

The color rendition becomes unclear, as the edge and thin lines of the subject are colored, if a chromatic aberration remains. The good color rendition is not always owing to the characteristics of glass materials and multi-layer coating alone.
Further, as this lens is free from restriction of back focus, distortion is very well corrected to practically nothing and astigmatism that is always a problem with wideangle lens is also small.
Contrary to the small astigmatism, the curvature of field remains a little, but the balance of aberrations is adequate if considering that the "NIKONOS" is a camera without a rangefinder. The curvature of field will not be a problem as it is concealed in the depth of the subject and a great depth of field, because, in a camera without a rangefinder, the aperture stop is stopped down in most cases. The lens with large astigmatism is not favorable in actual shooting even if it has better resolving power in the chart test because it causes image disturbance on both ends of depth of field.

As this lens is amphibious (for both on land and underwater), I will comment on the underwater performance, too.

It is noted in every introductory book on underwater photography that the subject looks 1.33 times larger under water than in air caused by the refractive index of water (1.33), picture angle gets narrower, and the focusing should be made by the distance estimated under water with the eye, but in addition, the change of aberration cannot be neglected.
In case that the water-tight window is made of a parallel plane glass just as this lens, it is known that the spherical aberration and astigmatism don't change but chromatic aberration and pincushion distortion are generated in the periphery of the picture frame. This distortion is generated by the front parallel glass in principle and the degree becomes larger in proportion to the picture angle of the lens. As the situation can be checked when you look into a water tank, please try it if you have a chance to go to an aquarium. If you observe the inside scene of the water tank of which window is made of parallel plane glass, you will find that the inside scene is distorted and colored at the far side of the tank.
Because of that reason, regarding the lens wider than 35mm for "NIKONOS", separate lenses were prepared for use on land and underwater respectively such as "UW NIKKOR 28mm f/3.5 (exclusively for underwater)" and "LW NIKKOR 28mm f/2.8 (exclusively for on-land use)" (1983), for instance.

III. Lens performance

The outlook feature of this lens is, above all, the aperture knob and the focusing knob arranged at left and right side of the lens barrel respectively and the aperture scale and the distance scale arranged in the front of the lens.

Manipulating the "aperture knob" arranged at the left side looked from the front, the "aperture scale" rotates as the aperture stop opens and closes, and, at the same time, the pair of depth-of-field indicators (red) move and they hold the "distance indicator" (white triangle ( IMG ), top of white arrow with the current model) in-between in the distance scale window, thus the "range" of depth of field is shown in the "distance scale" window. Manipulating the "focusing knob" arranged at the right side looked from the front, the "distance scale" rotates, and the black figure (m) (or red figure (ft.)) of distance estimated by the eyes should be set to the distance scale indicator.

This mechanism (depth-of-field indicator) is not only interesting to look but very helpful, as the depth of field is confirmed at a look and make it easy to focus with estimating the distance by the eyes.
"Focusing by the estimation of the distance by the eyes" may be kicked upstairs as it sounds to be difficult to the people of auto focus age but it is not so difficult once accustomed, since from infinity to 3 m enters in the range of depth of field (see figures) if 35mm lens is stopped down to f/8.
To get the knack of estimating the distance by the eyes, you should develop the sense of distance in everyday life from the stride of your own or the dimensions of a room, and you can judge the distance from the size of the subject in the viewfinder frame, in case of taking a person, for instance, "full stature in horizontal framing is 3 m apart" or "full stature in vertical framing is 2 m apart" with reference to the memory of the distance.

Lastly, I wish to discuss the imaging characteristics of the lens with example photographs. Please excuse me that I prepared only on-land examples as I am not much experienced in underwater photography.

(Example photograph 1.) is an urban scenery. It is taken with the aperture stopped down to f/11 since the imaging of this kind of building requires the definition and uniformity over the frame.
With the aperture fully open at f/2.5, flares are discerned but at f/5.6 the image of the central part of the frame is sharp without flares.

At f/11, the sharpness is better and the entire frame is almost uniform.
However, it provides soft imaging with "somewhat rounded profile even in the sharpness" at f/8~11, quite different from "crisp and high contrast image" of "AI Nikkor 50mm f/2" stopped down to f/5.6 discussed in Tale Two.
That's a delicate difference that cannot be discerned on the monitor screen of a personal computer, but we have the impression that this lens provides smooth imaging.
As the lens used this time is a relatively new type with multi-layer coating, obvious ghost images and flares are not found even the strong summer sunshine is reflected by the window glass of the building.

(Example photograph 2.) Flowers taken in the distance of about 1.2 m. Aperture is stopped down to f/5.6 and the background is defocused in smooth gradation. At frequently used stops, f/5.6~8, this lens provides an excellent defocus performance.
As it is difficult to make a good estimate of the distance even with a 35mm lens in the distance as close as this, focus bracketing, to shoot a few frames with shifting the focusing distance bit by bit will be practically effective.

Looking back the past tales, I found that we tended to introduce wideangle to normal lenses more often in line with the plan of this series that we, two writers (SATO and OHSHITA) take up our favorite lenses at random. In the Tenth Tale, my next turn, I will introduce one of typical telephoto lenses. Please expect.