Review of the Rollei Sonnar 40/2.3 fixed lens, adapted for mirrorless cameras

Material especially for Radozhiva prepared Rodion Eshmakov.

Adapted Rollei Sonnar 40/2.3.

Adapted Rollei Sonnar 40/2.3.

Rollei Sonnar 40/2.3 – fixed lens for a tiny full-frame rangefinder camera Rollei XF35, which can very rarely be found at flea markets in the post-Soviet space. The lens was produced in Singapore in a single version under the brand of a German company Rollei under license from Carl Zeiss Oberkochen (Germany) from 1974 to 1980, while it is faster than the Rollei Sonnar 40/2.8 HFT for the Leica M system. This article presents the Sonnar 40/2.3, adapted for use with system mirrorless cameras .

Specifications:

Optical design – 5 lenses in 4 groups (1 element made of highly refractive glass, n>1.8), “Sonnar”;

Drawing of the optical design of Sonnar 40/2.3.

Drawing of the optical design of Sonnar 40/2.3.

Focal length - 40 mm;
Relative aperture - 1: 2,3;
Field of view - 56 °;
Rear focal line - ~ 27 mm;
Aperture (factory version) – behind the lens, 2 Γ-shaped blades;
Aperture limits (factory version) – 1:2.3 – 1:16 (controlled by the camera);
Focusing - manual;
Minimum focusing distance (factory version) – 1 m;
Thread for filters – 46×0.75 mm;
Features: fixed lens, no aperture control ring.

Design and adaptation

For adaptation, a lens from a faulty Rollei XF35 camera with perfect optics was found. The lens was removed from the camera along with some of the body parts that were planned to be used in the refurbishment to maintain a recognizable appearance - I really liked the original design of both camera and lens.

The main problem was the selection of the aperture and the method of controlling it: the lens has a short rear segment and a small lens block, which makes it difficult to remove the aperture control ring and secure the drive. Ultimately the lens unit was installed reversibly into the empty lens unit housing Industar-61 with a ten-blade diaphragm. At the same time, parts of the I-61 aperture drive were also used in the new lens. Ultimately, the lens block was fixed in a part turned by a lathe with an M42 thread and placed in a high-quality macrohelicoid M42-M42 17-31, providing a short minimum focusing distance of approximately 20 cm.

In the adapted lens, the aperture is controlled by turning a ring with a thread for the filter, and focusing is done by the macrohelicoid mechanism. Thin adapter rings can be used as a shank, in my case - M42-NEX. The photodiode of the exposure meter was removed during adaptation, and the film sensitivity selector ring was left as decorative. Appearance of the adapted Sonnar 40/2.3, including in comparison with other Sonnar type lenses - Jupiter-3 50/1.5 и Zorkiy BK 35/2.8 (Jupiter-12) – presented below.

The adapted Sonnar 40/2.3 retained the compactness of the original lens, while receiving a new round aperture and a radically shorter minimum focusing distance.

Optical properties: some technical information

Previously, it was not widely known who is the author of this particular version of the Sonnar optical design - patents include Ludwig Bertele, the inventor of the Sonnar circuit as such, does not have this lens. As it turned out, the calculation of this lens, Sonnar 40/2.8 and many others was carried out by another famous German optician - Erhard Glatzel, who became famous for calculating the record-breaking lens Carl Zeiss Planar 50 / 0.7. However, apparently, Glatzel and Bertele were colleagues for some time, as evidenced by the presence of joint publications.

The five-element optical design of the Sonnar 40/2.3 lens, the design of which has not previously been published anywhere, is shown in patent US 3994576 (1976) in a version with aperture F/2.4 (example 8). Most likely, the production lens had differences from the one shown in the patent, but simulation of the published circuit shows a high level of agreement between the observed and theoretical results.

Optical design of the f/2.4 lens from US patent 3994576 (1976) indicating the brands of optical glass.

Optical design of the f/2.4 lens from US patent 3994576 (1976) indicating the brands of optical glass.

As you can see, in this lens, unlike, for example, the six-element Sonnar 50/2 Ludwig Bertele, there is no glued triplet in the front part - it was replaced with a doublet with an air gap (lenses 2 and 3), which is more advantageous in terms of correcting aberrations and permissible in the era of developed anti-reflective coatings.

A curious feature of the lens is the very small air gap between the third and fourth lenses, where the lens aperture diaphragm is located. Obviously, it is very difficult to place the iris diaphragm in this place, therefore the serial Sonnar 40/2.3 uses a behind-the-lens diaphragm. This technical solution, however, in practice does not lead to a catastrophic increase in vignetting with aperture.

The lens makes full use of modern (even by today's standards) optical materials - heavy lanthanum crowns and flints. At the same time, the serial lens is made using outdated versions of optical glasses containing toxic elements - cadmium, arsenic and lead, as evidenced by X-ray fluorescence spectra.

X-ray fluorescence spectrum of the front lens of the objective lens. Detection of Zr is an instrumental artifact.

X-ray fluorescence spectrum of the front lens of the objective lens. Detection of Zr is an instrumental artifact.

X-ray fluorescence spectrum of the rear lens of the objective lens. Detection of Zr is an instrumental artifact.

X-ray fluorescence spectrum of the rear lens of the objective lens. Detection of Zr is an instrumental artifact.

The qualitative elemental composition of the glass of the front and rear lenses of the serial lens corresponds well to the brands of optical glass stated in the patent. Thus, Ba, Sr, La and As are detected in the front lens, which corresponds to lanthanum crown glass GOST/IPZ/LZOS STK12, and the rear lens contains Ba, La, Sr, Pb, As, Cd, Zn, which corresponds to lanthanum flint, the closest analogue of which in the GOST/IPZ/LZOS catalog is STK9 glass, used in the Industar-61 lens.

The use of modern optical materials has made it possible to increase the angle of view of the lens by 30% in comparison with the Sonnar 50/2 with a similar aperture ratio and a simpler design. In comparison with the old Biogon 35/2.8 https://radojuva.com/2020/12/zorkiy-bk-2-8-f-3-5-sm-p-kiev-zorkij-35mm/ Ludwig Bertele Sonnar 40/2.3 has ~2/3 EV more aperture with a 7% smaller field of view angle, almost 4 times larger rear segment and a simpler design.

Highly refractive glass made it possible to better correct spherical and spherochromatic aberration, due to which the lens has fairly high sharpness and resolution.

Graph of longitudinal aberration of an f/2.4 lens from US patent 3994576 (1976).

Graph of longitudinal aberration of an f/2.4 lens from US patent 3994576 (1976).

As compared with Jupiter 12 и Jupiter 3 This simpler lens also copes better with distortion, which does not exceed +1.5%.

Graph of field curvature and astigmatism of an f/2.4 lens from US patent 3994576 (1976).

Graph of field curvature and astigmatism of an f/2.4 lens from US patent 3994576 (1976).

The only significant drawback of the lens is the extremely poor control of astigmatism at the very edge of the field. Very many 40 mm lenses based on circuits triplet, Tessar and Sonnar have similar astigmatism and field curvature curves.

Graph of f/2.4 lens distortion from US patent 3994576 (1976).

Graph of f/2.4 lens distortion from US patent 3994576 (1976).

The following are graphs of the lens contrast transfer function for the spectral range of 400-700 nm (based on the spectral sensitivity function of the Sony A7R2 matrix) at F/2.4 and F/8. At an open aperture, the lens resolution is 60 lines/mm, at F/8 – 100 lines/mm.

Calculated lens contrast transfer function from US patent 3994576 (1976) at F/2.4.

Calculated lens contrast transfer function from US patent 3994576 (1976) at F/2.4.

Calculated lens contrast transfer function from US patent 3994576 (1976) at F/8.

Calculated lens contrast transfer function from US patent 3994576 (1976) at F/8.

The serial Sonnar 40/2.3 lens most likely uses a fairly simple anti-reflective lens coating (1-2 layers), applied using a physical method. The coating on each surface is the same, which leads to significant distortion of the transmission spectrum and color veiling in difficult lighting conditions. However, this coating scatters less light than the single-layer chemical coating on Soviet optics.

Transmission spectrum of the Sonnar 40/2.3 lens.

Transmission spectrum of the Sonnar 40/2.3 lens.

As you can see, a fresh look at the optical design of the 30s has borne fruit. Sonnar 40/2.3, based on calculations by Erhard Glatzel, is a simple and compact lens from which you can expect an acceptable level of optical quality.

Optical properties: sharpness test of lenses BK 35/2.8 (Jupiter-12) and Sonnar 40/2.3

Far-field image quality comparisons were made using a full-frame Sony A7s camera. When changing the aperture number, refocusing was carried out in the central area. The following are 100% crops from the center of the frame, edge and corner.

100% crop center of frame.

100% crop center of frame.

100% crop edge of the frame.

100% crop edge of the frame.

100% crop angle of the frame.

100% crop angle of the frame.

It is curious that in the central area of ​​the image the BK 35/2.8 lens (Jupiter-12) turns out to be sharper than the Sonnar 40/2.3 up to F/4. Indeed: spherical aberrations in Jupiter-12 are corrected very well. In the range from F/4 to F/8, the Sonnar 40/2.3 performs better in the center due to its smaller secondary spectrum.

At the edge of the frame, both lenses behave in a similar way, but in the corner of the frame, the leadership in image quality is definitely with the Sonnar 40/2.3. But this victory goes to him not due to better correction of aberrations, but because of a much larger back segment: the image-forming beams of the Sonnar 40/2.3 have a much more telecentric path than those of the Jupiter-12, which in the case of a digital camera with a thick matrix filter is very greatly affects the image quality across the field. Based on the results of modeling the optical design of Jupiter-12, its image quality in the corners of the frame should be significantly higher than that of Sonnar 40/2.3. For more details on the effect of a filter on image quality, see here.

Optical properties: comparison of the design of the lenses BK 35/2.8 (Jupiter-12) and Sonnar 40/2.3

Jupiter-12 and Sonnar 40/2.3 are lenses similar in application scenario. One of the advantages of old Sonnar-like lenses is considered to be bokeh, which is very pronounced when shooting small objects with both lenses in question. Below are examples of photos taken with a BC 35/2.8 at F/2.8 and a full-frame Sony A7s camera.

Then - paired photos with Sonnar 40/2.3 at an open aperture.

In the central area of ​​the frame, the lenses behave in a similar way, but at the edge of the field the bokeh differs in approximately the same way and to the same extent as in the case of comparisons Jupiter-3 and Minolta Rokkor PF 58/1.4.

User experience

Strictly speaking, there are not so many lenses with a focal length of 40 mm that occupy a compromise niche between the usual 50 mm standard lenses and 35 mm wide lenses. There are old “pancakes” from SLR cameras - Hexanon 40 / 1.8, Canon EF 40 / 2.8 STM, Pentax 40 / 2.8 – but all of them are no longer so compact on a mirrorless camera. There are also autofocus solutions for modern mirrorless cameras from Viltrox, Nikon, Sony - but these are optics of a completely different level. There are few manual “classic” 40 mm lenses, and there are even fewer more or less high-quality and fast-aperture lenses among them.

The Sonnar 40/2.3 is a unique lens in its own way: it provides fairly high-quality images compared to many older ones 35-45 mm lenses, but at the same time it has a characteristic, recognizable and attractive design, like the more famous lenses bearing the same name. In terms of the picture and parameters, it is very difficult to find an alternative to the Sonnar 40/2.3 lens. Lenses of class 40/2.8 with a Tessar design that are similar in design are much worse in quality, and lenses of the “double Gauss”/Planar class 45/2, if better in quality, are not at all the same in design. Perhaps the only noticeable drawback of the lens is the anti-reflective coating, which noticeably distorts the color, especially with color veiling in backlight. I am more impressed by lenses in which the coating varies on different surfaces by highlight color, which helps to neutralize both the transmission spectrum and the color of the veil.

The following are examples of images taken with the Sonnar 40/2.3 and the full-frame Sony A7s camera.

Conclusions

The Sonnar 40/2.3 is an excellent lens in terms of its combination of properties both for its Rollei XF35 camera and for modern mirrorless cameras. An excellent example of a successful rethinking of an old optical design by Ludwig Berthele using modern materials and approaches to optical design. The lens is relatively difficult to adapt and is rare, but if that doesn't stop you, the Sonnar 40/2.3 is worth a try.

You will find more reviews from readers of Radozhiva here.

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Comments: 2, on the topic: Review of the Rollei Sonnar 40/2.3 fixed lens, adapted for mirrorless cameras

  • Vladimir

    Thanks, very interesting!

  • Alexander

    Thank you very much for your review. Olympus and Kosina had very good 40mm lenses.

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