NIKON

Optical Design value Analysis - NIKON AF-S NIKKOR 50mm f/1.8G [No.017]

2022-06-24

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

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

Even if you look it 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 is a project to enjoy the history of NIKON lenses on my own.

First of all, as of 2020, the following NIKON 50/58mm lenses 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
Eight lenses…that's a menace. Moreover, each of them seems to have a different optical system.

It is understandable if the motors, drive mechanisms, electromagnetic aperture, etc. are different, but why do they even bother to change the optics… This is partly because it is the dawn of the mirrorless era, but how much does NIKON love the 50mm?

This is the second installment of the series. In the first installment, we started our analysis with the f/1.8D, which is a basic double-Gaussian configuration, but this time the optical system specs are the same: f/1.8G.

The general public may think, "Aren't these two lenses the same, with only mechanical differences such as the focusing mechanism and electromagnetic diaphragm?" I also thought they were the same until I looked into it this time.

I also thought they were the same until I looked into it this time.

To my horror, Nikon has designed completely different lenses.

The optical system of the f/1.8D is based on the optical system released in 1978, as shown in the patent documents and information on NIKON's website.

On the other hand, the f/1.8G was released in 2011, so there is a difference of about 30 years in the development period. How this difference will be reflected in this article will be the real pleasure of this article.

Translated with www.DeepL.com/Translator (free version)

Private Memoirs

Although each company sells lenses with a simple double-Gaussian 6-element configuration, we did not intend to feature them because they are not unique.

This is proof of the excellence of the double Gaussian itself. However, in order to take the liberty of analyzing the history leading up to the Z58mm f/0.95, I will make a special exception for NIKON's 50mm f/1.8.

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NIKON AF-S NIKKOR 50mm F1.8G

Document Survey

Now, I was worried about whether I would be able to find a patent for Gaussian-type optics, since each company issues a variety of patents, but I was able to successfully find one. I was afraid that there would be too many similar patents….

Now, let's assume that the design value is Example 1, which seems to have good appearance and performance from the found patent application 2011-175123, and 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

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

It consists of 7 elements in 6 groups, with one additional convex lens on the image sensor side of the symmetrical Gaussian. The sixth lens is an aspherical lens.

Compared to the f/1.8D, it is quite luxurious.

Longitudinal Aberration

Graphs of spherical aberration, image surface curvature, and distortion

Spherical Aberration , Axial Chromatic Aberration

Both spherical aberration and axial chromatic aberration have been reduced by about half compared to the f/1.8D.

Spherical aberration is probably removed by the effect of the aspherical lens.

Axial chromatic aberration is corrected by increasing the number of glass types. f/1.8D was designed with two types of threatening glass types, but this f/1.8G lens uses the same glass types for the second and fifth lenses, but the other types are different.

Image Surface Curvature

The image curvature is about the same compared to the f/1.8D, but it seems a little worse considering the suppression of spherical aberration?

Distortion

The distortion is numerically in the small range, but it is worsening in the negative barrel shape direction. If it were perfectly symmetrical, it would be much less, so I think it is because the symmetry has been shifted.

Chromatic Aberration

Similarly, the magnification chromatic aberration is small in absolute value, but it is worse than that of the f/1.8D. This may be due to the shift from a symmetrical structure.

Transverse Aberrations

(Left)Tangential direction, (Right)Sagittal direction

Let's look at it as a lateral aberration.

The aberration of the reference ray (d-line: yellow) is about half that of f/1.8D in both tangential and sagittal aberrations.

Spot Diagram

Spot Scale 0.3 (Standard)

Here are the results of the optical simulation. Let's start with the spot diagram.

Since this is a 2011 product, the spots are quite docile.

Spot Scale 0.1 (Detail)

In the enlarged detail check scale, the V-shaped effect caused by sagittal coma flare becomes stronger when the image height exceeds 18 mm at the periphery of the image.

MTF

Maximum Aperture F1.8

Finally, let's check the results of the MTF simulation.

Compared to the f/1.8D, the MTF is improved by about 10 points from the center to the mid-range of the image height of about 18mm.

Small Aperture F4.0

Although high enough, image plane fluctuation occurs and is not much different from that of the 50f/1.8D.

Each aberration has been reduced by about half, and the performance at wide aperture Fno has improved sufficiently, but the performance difference at small apertures seems to be almost none or slightly worse…

However, the feeling that there is no difference is the terrible underlying power of the symmetrical arrangement type. Even with 30 years of time and technology refined over that time, only this level of improvement can be achieved.

Of course, if the size of the lens is made huge like Sigma's Art lenses, the performance will be even better, but if the size is kept at the same level, only this level of improvement can be achieved.

If it is not good enough, people will say that the old lenses have a better taste in the maximum aperture f/n ratio, and the heart of the person who designed this lens has probably been shattered by the Gaussian.

However, if we compare it to the progress of computers, a 30-year difference would be as dramatic as a tricycle becoming an F1 car, so progress in lenses is slow at any rate.

The performance of the lens is quite sharp from wide open, so it will satisfy the needs of those who want a lightweight, high performance lens.

Conclusion

Compared to the old 50f/1.8D, the policy was to improve the maximum aperture performance while keeping the weight and size Up in check, and I think it was an appropriate balance for the first renewal in 30 years.

For those who are tired of the ultra-high-performance and ultra-heavy lenses of recent years, this lens is just right for owning as a "light and playful lens" that retains the old-lens-like flavor while still being able to autofocus effectively.

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NIKON AF-S NIKKOR 50mm F1.8G

Sample Picture

Example photos are in preparation.

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

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