OLYMPUS

Optical Design value Analysis - OLYMPUS Zuiko 50mm F1.2 [No.001]

2022-02-27

Olympus Zuiko 50mm F1.2 performance analysis and review article.

We estimate the design values of the optical system based on patent information and actual photographic examples, and analyze the lens performance through simulation.

Professional lens designer Jin Takayama carefully unravels design information such as optical path diagrams and aberration characteristics, which are rarely seen by the general public, and provides deep and gentle explanations.

Please enjoy the special information that can only be read on this blog.

Overview

The zuiko 50 1.2 is the largest aperture ratio lens in the Zuiko lens series for Olympus OM mount.

It was released in 1983 and there are early and late models. I bought it new around 2000.

In the latter half of the 1970s, competition among companies to produce lenses with a focal length of 50mm resulted in the release of many lenses in the Fno1.2 class.

In film photography, the ISO sensitivity could not be changed as easily as with digital cameras, so a bright Fno was a "certainty of justice" in itself in order to increase shutter speed.

Private Memoirs

When I purchased this lens around the year 2000, the era of digital cameras was dawning.

However, full-size digital cameras were still the stuff of legend, and film cameras were still very much in use.

In the camera section of Yodobashi Camera, the Olympus OM4 film SLR camera was proudly displayed.

At that time, I remember purchasing this Zuiko 50 1.2 for around 50,000 yen.

Even now, it seems to cost around 50,000 yen at auctions if it is like new. (As of 12/2019)

It seems that old lenses from the pre-film era are called classic lenses or old lenses.

Since this lens was first released around 1980, it must be in the category of old lenses. Hardcore old lens enthusiasts value lenses as old as pre-war ones, so this lens must be a "young lens.

In this blog, lenses from the film era are treated as old lenses, so this Zuiko 50 1.2 lens is also treated as an old lens.

Document Survey

As of this writing (2020), Japanese patent documents up to around 1970 have been digitized and published.

Although the patent documents from the 1970s have been digitized, they are still stored as scanned images of paper data and cannot be searched by keywords.

However, they are somehow categorized by manufacturer, so if you search all the patent documents related to Olympus lenses from the late 1970s, you can find several documents with specifications of 50mm focal length and Fno1.2.

Since the number of patents for 50mm lenses seems to be high in relation to the total number of patents for optical systems filed by Olympus, it seems that the company was putting a lot of effort into development. The patents of this era are not handwritten, as expected, but they were printed out and then scanned back into electronic data, so the images are rough and difficult to read…

The example of the patent shows the focal length as 100mm, but some manufacturers describe the examples of patents for optical systems by proportionally multiplying the entire system so that the focal length becomes 100 or 1, so it is more reliable to estimate from the angle of view.

Now, based on the age of the patent and its performance, it seems that there are only two choices for the patent, but I will reproduce the design data below, assuming that they commercialized JP 53-69031, which seems to have good performance based on my intuition.

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 of the zuiko 50 1.2 lens, which is composed of 7 elements in 6 groups and does not employ an aspherical lens.

The configuration is a large-aperture Gaussian type with one additional positive lens element on the image side of the double Gaussian type, which is the standard configuration for SLR standard lenses.

Other companies have designs with an eight-element construction, adding one lens element to the front and one to the rear, but Olympus seems to have decided that one lens element on the rear side was sufficient.

At the time of the launch of this lens, many patents were being issued for the use of aspherical lenses, but this lens was designed using only spherical lenses.

At that time, design was already being done by computer, but if today's computers are like jet planes, then the computers of that time must have been like mosquitoes.

What kind of hardships did they have to go through to design in such a development environment?

If you look closely at the optical path diagram, you can see that the light flux on the axis (center) is very thick due to the large aperture, and the light flux at the periphery is relatively thin, resulting in severe vignetting.

This is different from the cosine quadrature attenuation that occurs with wide-angle lenses, and is unique to large apertures.

This loss of light amount is dramatically improved by using a smaller aperture.

Since this is a Gaussian-type lens with no aspherical surfaces and a large f/1.2 aperture, expectations are high for the appearance of severe aberrations typical of old lenses.

One of the excellent points of the Gaussian type is that it is possible to achieve high performance without using special materials, and this lens does not use any special materials worthy of special mention.

Longitudinal Aberration

Graphs of spherical aberration, image surface curvature, and distortion

Spherical Aberration , Axial Chromatic Aberration

Let's take a look at the performance of the zuiko 50 1.2, starting with spherical aberration. Although the Gaussian type lens has a large amount of spherical aberration in the full collection form for an optical system with a target configuration, it is quite well corrected considering the number of components and the large aperture of f/1.2.

Surprisingly, spherical aberration is suppressed to a large extent in the middle of the vertical axis at about 0.75. At the 1.00 position at the top of the graph, the aberration has a unique shape that seems to jump up.

It is a little strange that such a shape can be achieved with such a small number of components, but the jumping up at the 1.00 position at the top end is cut off when the aperture is stopped down, so it can only be seen when shooting at maximum aperture.

The aberration at the 0.75 position is suppressed, so when you stop down the aperture, you can see that the resolving power increases sharply. The way this aberration is left in is a technique, a so-called taste.

For the axial chromatic aberration, the c line (red) and the g line (blue) are left overlapping. This is a technique to make it less noticeable by making it purplish, as it would be noticeable if it were a bright red blur. However, I think the image is well-corrected for a 1.2 f-number and this composition.

Image Surface Curvature

Despite the large aperture, image surface curvature is well corrected up to the middle of the image. Since the spherical aberration is fully under-corrected, the image plane should be slightly under-corrected so that the entire image is in uniform focus.

Distortion

Distortion is slightly tull, as is typical of Gaussian types, but the absolute value is small and not a problem.

Chromatic Aberration

Because of its symmetrical structure, the magnification chromatic aberration is nicely corrected. It is almost ideal.

When shooting with a wide aperture, lateral chromatic aberration is not noticeable, so there is not much benefit to be gained. However, a small amount of lateral chromatic aberration is important because it greatly improves MTF at small apertures.

Transverse Aberrations

(Left)Tangential direction, (Right)Sagittal direction

Although there is a huge sagittal halo at the periphery of the light beam, it is twisted in the opposite direction to the inclination near the main light beam, so the total focus is mysteriously balanced, which is a standard way of producing aberrations at large apertures.

However, I'm sure it's just a coincidence, but I wonder how people in the past thought of this…

This halo is the flavor of so-called old lenses, and as you stop down the lens, the aberration is cut, and the resolving power increases dramatically.

Spot Diagram

Spot Scale 0.3 (Standard)

Sagittal flare is the effect of lateral expansion at high image heights, and if there is a point light source in the periphery, it will form a V-shape like this, so be careful when shooting night scenes and starry skies.

Spot Scale 0.1 (Detail)

Here is a more enlarged view with the scale changed. It is difficult to compare modern lenses unless they are shown in this scale, but it is a little tough to apply it to this old lens.

MTF

Maximum Aperture F1.2

At maximum aperture of f/1.2, the top of the MTF peak is low, but the position of the MTF peak is quite consistent. You can observe that the focus plane is flat when you shoot in a way that shows the depth direction.

Small Aperture F4.0

It has a bright aperture of f/1.2, and when you stop down to f/4.0, the MTF improves dramatically and is comparable to modern lenses.

From a photographic point of view, the resolution is too high to be interesting, but…

Conclusion

The Zuiko 50 1.2 is a wonderful design that can be enjoyed twice: the first time when the lens is wide open due to its unique way of retaining spherical aberration, and the second time when the lens is stopped down due to its improved resolution.

In recent years, as lenses have become larger and more powerful, it has become impossible to enjoy this kind of tasteful imaging.

It takes a lot of courage to buy an antique lens from before and after the war, but the zuiko 50 1.2 was sold new until around 2000, so it is readily available on the used market at a reasonable price.

It is a must-have lens if you enjoy aberrations as a taste.

Sample Picture

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

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