NIKON

Optical Design value Analysis - NIKON AI AF NIKKOR 50mm f/1.8D [No.016]

This is a performance analysis and review article on the Nikon Nikkor 50 1.8D.

You hardly know how the lenses work, how their performance differs from each other, and the specific differences between them.

Even if you look them up in magazines or on the Internet, you probably only find similar "word-of-mouth recommendations" articles like that.

In this blog, while researching the history of lenses and their historical background, we estimate lens design performance based on patent information and actual shooting examples, and analyze lens performance in detail from a technical viewpoint through simulations.

Professional lens designer Jin Takayama will carefully unravel optical characteristics such as optical path diagrams and aberrations that are not generally visible, and explain the taste and descriptive performance of lenses in a deep and gentle manner.

Please enjoy the special information that you can read only on this blog in the world.

Overview

In commemoration of the discovery of the NIKKOR Z 58mm f/0.95 S Noct patent documents, we have decided to proceed with a series of analyses of the NIKKOR 50mm.

In short, this series will be a project to enjoy the history of NIKON lenses on its own.

First of all, as of 2020, the following NIKON lenses with focal lengths of 50mm and 58mm are currently available.

  • AI AF NIKKOR 50mm f/1.8D
  • AF-S NIKKOR 50mm f/1.8G
  • AI AF NIKKOR 50mm f/1.4D
  • AF-S NIKKOR 50mm f/1.4G
  • AF-S NIKKOR 58mm f/1.4G
  • Ai NIKKOR 50mm f/1.2S
  • NIKKOR Z 50mm f/1.8 S
  • NIKKOR Z 58mm f/0.95 S Noct
  • 50mm /f1.2S is limited stock.

The total number of lenses is eight. Moreover, each of them seems to have a different optical system.

It would be understandable if, for example, the motor, drive mechanism, and electromagnetic aperture were different for each lens, but why go to the trouble of changing the optics as well?

Since this is the first installment of this series, we will start our analysis with the most basic f/1.8D with a double-Gaussian configuration.

The NIKON 50mm f/1.8 is generally referred to as a "bait lens," and if you are attracted by its cheap price and buy it on impulse, it has the power of a terrible curse that will drag you into a dark and deep lens swamp.

The official page of NIKON also has a summary of the development history of this lens, so please read it as well.

 Related article: Nikkor One Thousand and One Nights - Night 60

To summarize very briefly, the optical system of the NIKON 50mm F1.8D is based on the optical system of the Nikkor lens series known as the E series, which was sold with the Nikon EM, and was first released in 1978.

It is a fearsome lens that was produced for about 35 years, since its production seems to have ended around 2013.

Private Memoirs

Lenses with a simple double-Gaussian 6-element configuration are still sold by various companies, but we did not intend to feature them because they are not unique.

Conversely, this is proof of the excellence of double Gaussian lenses.

However, I will make a special exception for NIKON's 50mm f/1.8 lens in order to analyze the history leading up to the Z58mm f/0.95 lens.

I was also indebted to this lens when I was young, but since I had fallen into the lens swamp before using the 50mm f/1.8 in the first place, I had a light impression of it as "light and good when used occasionally.

That is, until I became an optical designer by profession…

Document Survey

The full name of the designer is listed on the Nikon's official website on the page of development secrets.

If we could only find the author's name, we could finish our search in one stroke. Assuming that the design value is Example 1, which seems to have good performance from JP 54-104334, let us reproduce the design data as follows.

Notes!

The following design values have been selected and reproduced from the appropriate patent literature and do not correspond to the actual product. Naturally, the data is not guaranteed, and I am not responsible for any accidents or damages that may occur by using this data.

Analysis of Design Values

Optical Path Diagram

Longitudinal Aberration

Graphs of spherical aberration, image surface curvature, and distortion

The above figure shows the optical path diagram of the NIKKOR 50 F1.8D.

This is a typical configuration of a modified double-Gaussian, with six elements in five groups and a basic six-element configuration with a separate junction on the subject side.

In the early days of perfectly symmetrical Gaussian lenses, it was difficult to suppress coma aberration while maintaining enough space (back focus) to accommodate the mirror of an SLR camera.

The new configuration of the lens on the subject side of the aperture (front-joint separation type) improved coma aberration and enabled the back focus to be secured to accommodate SLR cameras.

The reason for this is that the back focus was secured by slightly extending the focal length.

Achieving a focal length of 50mm for SLR lenses was a long-cherished wish of all companies.

After many twists and turns, and after much research and development by various companies, the front-joint separation configuration was discovered, and the 50mm focal length lens finally settled into its place as the standard for SLR lenses.

Furthermore, this lens is characterized by the gentle curve of the concave surfaces before and after the aperture and its thin design compared to ordinary front-joint separation type Gaussian lenses.

There is one more notable feature of this lens that is not apparent without looking at the design values.

That is that only two types of glass materials are used.

Even in this era, there must have been dozens of different glass materials, and a company as large as Nikon would have been allowed to design each lens with different materials to suppress aberrations.

How ascetic! Stoic! The fewer glass materials used, of course, the easier it is to procure materials, the lower the price, and the easier it is to manufacture in large quantities.

In short, this lens, which was designed to be cheap and compact, was a lens filled with the love of the designer who wished to provide the world with a good lens.

The reason why NIKON stubbornly continues to manufacture this lens, which may have already fulfilled its true role, is transparent.

Spherical Aberration , Axial Chromatic Aberration

The Gaussian type with a simple configuration has a full-correction shape with spherical aberration falling to the minus side to suppress focus fluctuations at wide aperture and small aperture.

Axial chromatic aberration is limited to about this level, but the F-line and c-line overlap at the tip of the spherical aberration, providing the ideal balance point between resolution and chromatic aberration suppression.

Image Surface Curvature

Image curvature is also left on the negative side to suppress fluctuations at wide open and small apertures. In the case of a Gaussian type, leaving the curvature intentionally is also a technique.

Distortion

Distortion is slightly barrel-shaped, but is characteristic of symmetrical Gaussian types and is in the small range in absolute value.

Chromatic Aberration

The characteristics of symmetrical lenses produce almost no lateral chromatic aberration.

Transverse Aberrations

(Left)Tangential direction, (Right)Sagittal direction

The spherical aberration/image curvature that is intentionally left out in the section on vertical aberration can be understood by looking at the transverse aberration.

When a large aperture lens is designed with a small number of elements, the amount of transverse aberration in the sagittal direction is enormous, but by leaving the vertical aberration to cancel out the focus component (halo), the balance between the central focus and peripheral focus is achieved.

Because of the small size and limited freedom of materials, there is a little bit of tangential coma aberration at a low image height of 6mm, but this is only noticeable when the lens is wide open because it improves when the lens is stopped down.

Spot Diagram

Spot Scale 0.3 (Standard)

As you can see in the lateral aberration, the spot is quite large, but this is the flavor that should be enjoyed.

The effect of the large sagittal aberration in the lateral aberration shows up in the spot. The spot is V-shaped at the edge of the image, so be careful when shooting stars and other objects.

Spot Scale 0.1 (Detail)

MTF

Maximum Aperture F1.8

The lateral aberration diagram shows that despite the enormous amount of aberration, the MTF peaks are reasonably consistent, indicating that the focus is balanced.

Small Aperture F4.0

Although the focus will shift to the minus side due to image curvature up to the middle part of the image height of 12mm, the effect of the aperture cutting the coma aberration will improve the height of the MTF peaks dramatically and the sense of aberration will be imperceptible.

Conclusion

It is an old lens that continues to be produced even today, but when I looked at its contents, I found that it is not a simple 6-element Gaussian lens, but a lens that is like a symbol of correct optical design, achieving thinness and reduced glass types within a small number of degrees of freedom.

I could understand the meaning of NIKON's careful production of this lens for more than 30 years, and for the first time at my age, I felt that "this is what I mean when I say I am impressed.

Sample Picture

If you are looking for analysis information on other lenses, please refer to the table of contents page here.

-NIKON