Ultra wide-angle lens for use in a SLR
At the end of 1959 when the Nikon F was unveiled, the Nikkor-O 2.1cm f/4 taken up in the first tale of The Thousand and One Nights was marketed, so it is said that a Nikon F system has had the ultra wide-angle lens from the beginning. This lens was a masterpiece of a symmetry-type designed for a rangefinder camera as SATO-san described in the first tale, but when attaching the lens to the Nikon F, there was inconvenience in such a way that a reflex mirror has to be put to a mirror-up position and an external finder was required to shoot.
The external finder was also required as a matter of course when photographing with a rangefinder camera like the Nikon SP, so it didn't matter in this sense. But, it was quite natural that a SLR user wanted to use a SLR finder when photographing with a SLR camera and that's why a merit was born in the SLR, not in the rangefinder camera. "Want to develop an ultra wide-angle that permits to photograph as we see". This was a determination addressed to the user in general all over the world and also it might be a determined will on the designing/developing side at Nikon.
According to the optical design report archived by design/development members, developing an ultra wide-angle lens was started about that time of designing a 24mm wide-angle lens. After a few years later, what was accomplished was the Nikkor-UD Auto 20mm f/3.5 that made its debut in 1968. To a 21mm focal length of an ultra wide-angle lens for use in a S-type rangefinder camera and the SLR, the focal length of this new lens was shortened to 20mm by 1mm, which might be in view of the 24mm lens. A wide-angle lens line for the S-type rangefinder camera comprised 35mm, 28mm, 25mm, and 21mm, whereas a lens line-up for the F-type SLR is 35mm, 28mm, 24mm, and 20mm and its focal length is getting shorter to 24mm from 25mm and 20mm from 21mm by 1mm. This was aimed at improving unbalance in which a difference in a focal length between 28mm and 25mm was small. The difference in the focal length of the wide-angle lens by 1mm brings about a big difference in a lens depiction and moreover, it's quite difficult to design and produce such the wide-angle lens with 1mm difference in the focal length. Venturing to shorten the focal length by 1mm was a decisive courageous decision on behalf of the user.
Now, FIG.1 shows a view of this lens construction, wherein, when taking a look at this lens as compared with that of the Nikkor Auto 24mm f/2.8 introduced in the tale fourteen, don't you have a feeling the lens construction might look alike somehow ? The reason is that these two lenses were designed by Mr. SHIMIZU, Yoshiyuki about the almost same time. At a time when a computer was not so developed as nowadays, wizard SHIMIZU-san didn't work on and accomplish two lenses with different specifications about the almost same time for nothing.
Adherence to a filter size
This Nikkor-UD Auto 20mm f/3.5 was a very excellent lens. Actually, flare on the periphery of the lens was small and distortion was next to nothing. A precise depiction of a subject in detail looks like something impressive even when observing an end result positive film through a loupe. However, only one thing was found unsatisfactory in this lens. It was a lens size. The Nikkor-o 2.1cm f/4 was so compact as if a user forgot a presence of this lens in use when attaching it to a camera body, whereas this 20mm f/3.5 was a just little bulky with 69.5mm in overall length and 390g in weight and moreover, this lens had a 72mm filter attachment size.
Staff members in Nikon F designing/developing considered consolidating an interchangeable lens filter attachment size with 52mm and 72mm from the beginning of the lens development.
They planned to adopt a 52mm attachment size for a daily use lens and a large 72mm attachment size for a limited special lens such a telephoto lens and the like unable to accept the 52mm attachment size in principle. At the time when the rangefinder camera was in its heyday, it was quite usual that filters supplied from each camera maker were different in attachment size respectively even for an interchangeable lens close to a standard lens, so the concept of consolidating the attachment size was epoch-making at that moment. From a viewpoint of this concept, adopting of the 52mm filter attachment size even for a 20mm ultra wide-angle lens must have been an object. The ultra wide-angle lens for the SLR camera was finally available on the market and then next object was to make a 52mm-attachment size filter for the ultra wide-angle lens.
All-new lens type
This dream of the 52mm-attachment size filter finally came to fruition as the (new) Nikkor Auto 20mm f/4 after six years later since the Nikkor-UD Auto 20mm f/3.5 made its sales debut through a process of several times of designing, product trials and improvement designing. An optical lens designer that was in charge of designing this lens was Mr. MORI, Ikuo. MORI-san was an authority on an ultra wide-angle lens design and designed a lot of ultra wide-angle lenses including the 13mm lens taken up in the tale nine of The Thousand and One nights.
This lens construction is as shown in FIG.2 and is completely different from the Nikkor-UD Auto 20mm f/3.5. A most outstanding feature was a forefront lens of a positive lens and had a unique construction at that time as the ultra wide-angle lens like the 20mm.
It was a prevailing thought that the ultra wide-angle lens took a long back focus and had a negative lens disposed as a first lens for converging rays of light at a wide angle. Under this prevailing theory conceived by many lens designers, Mr. MORI was determined to challenge this common theory and can realize an ultra compactness as never before with an all-new innovative lens type.
A patent was granted to an invention of this all-new lens invented by Mr. MORI as a matter of course and this lens was patented in Japan, U.S.A and Germany respectively. A lens in which a front lens element is a positive lens is what we call "a positive precedence-type" and this lens was a pioneer in the positive precedence-type ultra wide-angle lens. In view of the fact that many ultra wide-angle lenses thereafter available on the market employed the positive precedence-type, you can imagine how innovative this lens type MR.MORI invented was.
Image characteristics and lens performance
Ai Nikkor 20mm f/4
©2003/2004 Kouichi Ohshita
Ai Nikkor 20mm f/4
©2003/2004 Kouichi Ohshita
Referring to a sample picture, let's have a look at a picture depiction of the (new) Nikkor Auto 20mm f/4. As compared with the Nikkor-UD Auto 20mm f/3.5, the 20mm f/4 was twice as small as the 20mm f/3.5 and was thinner than the 50mm f/1.4 standard lens, but its performance can bear comparison. Furthermore, we have an impression in which a lens depiction of the 20mm f/4.5 was quite similar to that of the Nikkor-UD Auto 20mm f/3.5 on the surface in such a way that it might be confused with that of the 20mm f/3.5. This lens was not, by any means, a lens that sacrificed the performance due to compactness.
Sample 1 is taken with the aperture in full open and a hand-held camera since an indoor is not so bright. A texture of a building was sharply grasped. Regarding characteristics of this lens, sagittal coma flare is so low that this lens can be securely used from a maximum aperture. Also, you can tell from the sample that distortion was also low. But, at the maximum aperture, it can be seen that a light fell off around a film corner. As a light fall-off around a corner produces a tighten effect on a picture, however, I am wondering it cannot be categorically labeled as a shortcoming.
Generally, a characteristic of the ultra wide-angle lens is that an anterior object is depicted larger and a posterior object is taken smaller, so an object on the periphery is photographed as if it is pulled into around a film corner or it is distorted.
This phenomenon is called as perspective distortion and using of this distortion other way round successfully is a picture in which a deformed face of an animal companion like a rabbit and the like is photographed. To get a large deformation effect, the shorter the focal length such as 18mm or 14mm, the larger the deformation effect is produced, and this 20mm lens can depict, too. (See sample 2) Here is a tip for shooting; Approach a subject as close as possible, then stop down the lens to gain depth of field. This lens allows us to approach the subject only up to 30cm, so you might not expect a good result photo with a small subject. In this case, it will be convenient to use a close-up lens. The close-up lens * being usable is another benefit from the filter being sized to 52mm. This lens was reborn as the Ai NIKKOR at the same time when the Nikon F2A was launched on the market in 1977 and then switched to the Ai Nikkor 20mm f/3.5S of a new optical system in 1979.
This lens was a lens of which production was ended after a relatively short period, or five years since the lens was born with the Nikkor. On the contrary, a principle lens construction of the positive precedence-type ultra wide-angle lens has been succeeded to the Ai 20mm f/3.5 and further the Ai AF 20mm f/2.8D now available on the market which has been well accepted in the marketplace as the most compact and lightweight 20mm lens. This is a good example in which an innovative lens construction has been taken over unbrokenly.
The 20mm f/2.8D lens is so slim just like we can call a pancake lens and its physical size does not let you mind Furthermore, just in case, it works excellently with a good quality. This lens may be small, but it is not a lens to be dealt lightly with.
* Except for the close-up lens No.0, a light fall-off is noticed at a film utmost edge with the No.2, No.3T and No.4T attached. This film edge area might be out of a printed area when printing, but please make sure with a trial shooting.