OLYMPUS

Optical Design value Analysis - OLYMPUS Zuiko 21mm F3.5 [No.005]

2022-03-04

This is a performance analysis and review article of the Olympus Zuiko 21 f/3.5.

We analyze the lens performance through simulation by inferring the design values of the optical system from patent information and actual photographic examples.

Professional lens designer Hitoshi Takayama carefully unravels design information such as optical path diagrams and aberration characteristics, which are rarely seen by the general public, and explains them in depth and gently.

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

If you are looking for example photos, you can find them at the end of this article.

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

Overview

This is a Zuiko lens for the Olympus OM mount from the film era, and is a small aperture lens on the ultra-wide angle side.

Lenses for SLR cameras have the fate of being difficult to produce the performance of wide-angle lenses. As the focal length indicates, a wide-angle lens is a "short lens," but SLR lenses must have a "long back focus" to avoid interference with the mirror used to guide light into the viewfinder, a physical contradiction.

For this reason, the performance of SLR lenses has long been poor in comparison with rangefinder cameras such as Leica, and it can be said that overcoming this problem has been the evolutionary path of SLR lenses.

The reason for the high performance of wide-angle lenses for rangefinder cameras, so-called "Leica lenses," is simply that they do not require a mirror and can be designed with a short back focus, thus eliminating structural inconsistencies and allowing greater design freedom.

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

Private Memoirs

This Zuiko 21mm is my favorite focal length on the ultra wide angle side of the lens.

In my film days, my usual pattern was to equip myself with a 50mm and a 21mm.

When the focal length reaches 21mm, you are in the realm of ultra wide angle. From this point on, the world of exaggerated images beyond the angle of view of the human eye begins.

That said, the field of view is not as unusually wide as that of 18mm or 16mm, so 21mm is characterized by its ability to capture a wide field of view without unnaturalness.

The Zuiko 18mm F3.5 lens with similar specifications is also a good choice, but its first lens has an overhang that makes it a bit tricky to handle.

Document Survey

In order to investigate Zuiko 21mm, we scoured the patent literature and found JP 54-34234, which has a very similar cross-sectional shape.

In addition to the 21mm that we are analyzing here, we also found 24mm and 18mm, but due to poor printing or storage quality, the numerical values in some of the examples are unreadable, and many of the calculations cannot be performed properly.

I wonder if I should just give up since these are paper documents from more than 40 years ago…

We could try to redesign the lens based on the fragmented information, but that would take time, so we will finish the analysis of the Zuiko lens series with this lens and start analyzing other lenses.

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

Above is the optical path diagram of the zuiko 21 F3.5.

The lens consists of 8 elements in 7 groups, and aspherical lenses are not used. This is a typical wide-angle lens configuration with a relatively large lens on the subject side.

Although an ultra-wide angle lens, the first lens is a convex lens to reduce image curvature and distortion, which is common in older optical systems that do not use aspherical lenses.

The overall configuration is nearly symmetrical, so performance is expected to be good.

In addition, all companies have used this configuration as the basis for their ultra-wide angle lenses, and there has been a competition in development as to where to place the aspherical lenses.

To put it simply, if a large diameter lens on the front side, such as the first lens, is made aspherical, performance can be dramatically improved, but since the lens diameter is proportional to the price, the selling price will skyrocket.

Moreover, if the aspherical lens is too large, the processing itself becomes difficult, so the balance between price, performance, and the technology of the time comes into play.

This Zuiko 21mm lens does not use an aspheric surface, so it must be the crystallization of efforts to correct aberrations steadily using only spherical lenses.

By analyzing such lenses and looking at modern lenses again, we can appreciate the history of the development of lens design.

Longitudinal Aberration

Graphs of spherical aberration, image surface curvature, and distortion

Spherical Aberration , Axial Chromatic Aberration

Spherical aberration is well corrected, but axial chromatic aberration is large for a small aperture. Although red-side axial chromatic aberration is large, it overlaps with d-line and other colors at the upper end, so it seems to have been designed to be less noticeable at the wide open end.

Image Surface Curvature

Image curvature fluctuates greatly at the extreme periphery of the image, which is the upper edge of the graph, and a considerable amount of astigmatism is present. However, the balance is probably good for a wide-angle lens of this era. In the film era, the aspect ratio of film and printing paper was different, so the entire area of film was rarely printed. Therefore, the general public did not have the opportunity to look at the periphery of the film.

Today, it is commonplace to view every corner of the image on a monitor at equal size, so it has become a difficult time for people to see the entire image.

Distortion

Distortion is small enough, with about 2% barrel distortion at the maximum.

Since it is in the ultra wide-angle range, shouldn't there be a little more left? I think that is Olympus' commitment in that area.

Chromatic Aberration

Although the magnification chromatic aberration varies greatly depending on the image height characteristics, I think it is well integrated considering the number of wide-angle images and the size of the lens. The blue aberration is large at the periphery of the image at the upper end, but as with image curvature, it is an area that was not considered important in the sense of the time.

Transverse Aberrations

(Left)Tangential direction, (Right)Sagittal direction

The longitudinal aberration is well contained for a lens of this class, but the lateral aberration shows a large component of coma aberration in the tangential direction at image heights of 6mm to 12mm, and I am concerned about the impact of this aberration.

Spot Diagram

Spot Scale 0.3 (Standard)

Although the scatter is large, the light collection is weak and will not be a concern in actual shooting because of the wide angle and dark Fno. You can see why small apertures are so highly praised.

Spot Scale 0.1 (Detail)

MTF

Maximum Aperture F3.5

The center is quite high from wide open. This is probably the effect of suppressed spherical aberration. On the other hand, when the image height exceeds 12mm, it becomes extremely violent.

This is the effect of the coma aberration component, which can be seen in the lateral aberration. Since this is an ultra-wide angle, it should not be of much concern from a photographic point of view.

Small Aperture F4.0

It was a drop in the coma component up to the intermediate image height, so it improves firmly when stopped down. I wonder if this is the area that is highly praised.

The periphery of the image is degraded by image curvature, so although there is no improvement, there should be no problem in practical use.

Conclusion

As an ultra-wide-angle lens for SLR cameras of this era, the lens has considerable aberration in the periphery, but it has been designed with sufficient practical performance without using an aspherical lens, which is a testament to the high sense of the designers.

Since this is an ultra wide-angle lens, the sense of depth does not change much even when stopped down to about F8.0, so it can be used as an ultra wide-angle pancake lens if it is stopped down a little. In fact, when I tried this lens when creating the sample images, I found that there was no need to pan-focus, the image quality was excellent, and it was the best for casual strolling photography.

With today's full-size digital SLRs, ISO sensitivity can be raised as high as you want, so even if you use a darker Fno setting, it does not bother you in practical use.

Sample Picture

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

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