Industar-61 50/2.8: history of origin and development, adaptation, comparative review and test of the early version

Material on the lens prepared especially for Radozhiva Rodion Eshmakov.

The Industar-61 50/2.8 lens is one of the most common and mass-produced Soviet photo lenses, produced in a variety of versions from the 1960s to the 2000s and well known to many photography enthusiasts today. Despite the presence of a large number of different articles, reviews and comments related to the Industar-61 lens, it is hardly possible to find reliable information about the changes that the optical design of the lens has undergone over the years, as well as a comparison of different versions of the Industar-61.

This article consists of a review of the early rangefinder version of the FED I-61 52/2.8 lens, comparing its optical qualities with its predecessor lens – Industar-26m, as well as with one of the later versions of the lens – Industar-61 LZ MS  (1986). In addition, the article presents experimentally obtained data that clarify the sequence of origin and the essence of the development of the optical design of the lens.

Navigation

• Technical specifications
• Design and adaptation
• How many different I-61s were there? History of the creation and development of the optical design of the lens
• Optical properties of FED Industar-61: comparison with Industar-26m and Industar-61 LZ MS
• User experience. Photo examples
• Conclusions

Lenses Industar-61 (I-61) F=50 mm 1:2.8

About the history of lens development: here.

1. Industar-61L 2,8/50 (LZOS, mid-1960s) – M39 for rangefinder cameras, 10 petals.
2. Industar-61-Z – for SLR cameras with M39*45.2 mm mount (mentioned in the lens reference book of GOI ONTI A.F. Yakovlev, 1970, v.1, p. 71);
3. I-61 2,8 / 52 (FED) – M39 for rangefinder cameras, black and silver body, silver ring with depth of field scale. Ten-blade diaphragm. Early type of coating.
4. I-61 2,8 / 52 (FED) – M39 for rangefinder cameras, black and silver body, black ring with depth of field scale. Ten-blade diaphragm. Late-type coating.
5. Industar-61 L/Z 2,8/50 (LZOS, 1967-1971), M39 for Zenit SLR cameras. The name can also be indicated in Latin. The diaphragm is of a regular hexagonal shape.
6. Industar 61-M 2,8/50 (KMZ, since 1971) – M42 for Zenit SLR cameras, limited series, 5-blade aperture;
7. I-61-M 2,8/52 (FED, 1972) – M42 for Zenit SLR cameras, experimental;
8. Industar 61-A 2,8/52 (KMZ) – M42 for SLR cameras, with A-shaped mount, experimental;
9. INDUSTAR-61L / Z 2,8 / 50 (LZOS) – M42 for Zenit SLR cameras. The name can also be indicated in Cyrillic.
10. I-61 2,8/52 (FED) - for cameras FED-Atlas/FED-11, non-removable, with a central lock.
11. I-61 2,8/52 (FED) - for cameras FED-10, with a central lock.
12. I-61L / D 2,8 / 53 (FED) – M39 for rangefinder cameras, black case.
13. I-61L / D 2,8 / 55 (FED) – M39 for rangefinder cameras, black case.
14. Industar-61L / D 2,8 / 55 (FED) - M39 for rangefinder cameras, black body. Adapted for use with SLR cameras.
15. Industar-61L / D 2,8 / 55 (FED, 1992) – M39 for rangefinder cameras, experimental/small-scale production for FED-6TTL cameras.
16. INDUSTAR-61L/Z MS 2,8/50 (LZOS) – M42 for Zenit SLR cameras, with multilayer coating. The name can also be specified in Latin. It was produced until the 2000s.

Technical specifications (factory version):

[Source: Catalogue of lenses of GOI ONTI A.F. Yakovlev, 1970, pp. 73-78.]

Optical design - 4 lenses in 3 groups, "Tessar";

Focal length - 52.4 mm;
Relative aperture - 1: 2.8;
Estimated field of view angle - 45°;
Estimated frame format - 36×24 mm, covered - 44×33 mm;
Aperture – 10 blades, no preset mechanism, no clicks;
Aperture limits - F / 2.8-F / 22;
The back focal segment is 41 mm;
The minimum focusing distance is 1 m;
Thread for filters – 40.5 mm;
Camera mount – M39×1, working distance 28.8 mm.

Design and adaptation

The rangefinder version of the FED Industar-61 considered in this article is one of the earliest versions of the lens, the distinguishing features of which are a ten-blade diaphragm and an unpainted ring with a depth of field scale. In addition, this example has a coating, which is rare for FED Industar-61 lenses, completely analogous to that used with later lenses. Industar-26m.

Rangefinder versions of the Industar-61 lens can be used without special modification on mirrorless cameras using an adapter from the M39 thread of rangefinder cameras to the desired system. But this obvious option has its drawbacks. Thus, the main disadvantage of the rangefinder Industar-61 is that it has a very long minimum focusing distance (MFD) of 1 m, which is extremely inconvenient in a number of situations. Furthermore, the lens in the factory version is not compatible with tilt-shift adapters for mirrorless systems, although the optical design turned out to be admits and such a usage scenario.

There is an imperfect, but simple and economical way to modify the Industar-61 and Industar-26m, which allows solving the problems mentioned above: adapting the lens for SLR cameras. Probably the simplest option is to remake it for the Canon EF bayonet due to the shortest working distance among common SLR systems.

To adapt the I-61 to the EF bayonet, it is necessary to separate the lens block from the focusing mechanism. Then, disassemble the helicoid and grind off the protrusion from its outer part, which serves as a stop for the focuser, and cut off the inner part so that only the multi-start thread remains. Using screws on the outer part of the helicoid, you can secure the bayonet ring of the M42-EF type. The lens block is fixed in the inner part of the helicoid using the nut left after disassembly, which is screwed onto the tail part of the lens block body. To clarify the position of the infinity point, it is possible to change the position of the aperture control ring and the title ring of the lens.

The advantages of the given method are that there is no need for additional expensive or specially manufactured parts, the MDF is about 30-35 cm. The main disadvantage of the described method is the lack of fixation of the lens in the MDF position, the inconvenience of which, however, is compensated by the large angular travel of the lens block in the focuser (2-2.5 turns). The appearance of the Industar-61 rangefinder lens, including c EOS-NEX shift adapter, as well as in comparison with the adapted one Industar-26m and with lens Industar-61 LZ MS is given below.

Alternative approaches to lens adaptation are known. For example, transplanting the I-61 lens block into a helicoid Industar-50-2 – but this requires the presence of a faulty lens or damage to a working lens with similar properties. Another way is to use macrohelicoids, which, however, are themselves several times more expensive than the adaptable lens.

How many different I-61s were there? History of the creation and development of the optical design of the lens

Attempts to create a simple standard lens with 50/2.8 parameters based on the well-proven optical design of the Tessar were made in the USSR even before the Industar-61 lens appeared. Obviously, Soviet designers were aware of the experience of German opticians - in particular, Willy Merte from Carl Zeiss Jena - who had "overclocked" the four-lens design up to F/2.8 without using glass with rare earth elements. They were, of course, aware of the main problem of these lenses – the low level of contour contrast due to pronounced spherical aberrations. In connection with this, immediately after the war, still unknown (unfortunately) engineers at GOI designed the Industar-24 50/2.8 (11.1945) and Industar-26 (02.1946) lenses.

The latter, after minor recalculation (replacement of materials F4 → F2, LF-6 → LF7), was produced serially and in large quantities as a standard lens for rangefinder cameras FED and scale cameras Zarya. Moreover, a large-scale copy of the Industar-26 lens is the medium-format standard lens Industar-29 80 / 2.8. Probably, the key difference between the Industar-26m lens and its German prototype is a more “aggressive” mutual compensation primary and secondary spherical aberrations, due to which the contour sharpness increases (contrast at low frequencies ~10 lines/mm), but the resolving power of the lens decreases. Another disadvantage of the lens was the mediocre correction of astigmatism even in the middle of the working field.

A fundamental improvement in the optical properties of the I-26m 50/2.8 type lens was possible either by complicating the optical design by adding optical elements, or by using optical materials that were more advanced than the glasses of the 1930s, such as rare earth glass that appeared in the USA in the 1940s, primarily – thorium.

The new glasses had a refractive index value of 0.05-0.2 units (10-20%) higher compared to previously available materials with similar dispersion and had parameters from 650.570 (i.e. n~1.65 v~57) to 880.400 (n~1.88 v~40), which was absolutely mind-boggling in the 1940s and is still relevant today. There are major problems with ensuring the safety of industries associated with the extraction and processing of thorium minerals, purification and isolation of thorium compounds - a weakly radioactive element with rapidly accumulating (~60 years is the time it takes to establish secular equilibrium) highly radioactive decay products – subsequently forced the abandonment of the smelting of thorium glass.

The key to solving the materials science problem was the use of ThO2 instead of thorium oxide, which does not contain highly radioactive impurities (La-138 (not counted) lanthanum oxide La2O3 – also related to rare earth elements. The high melting point of La2O3 and its chemical activity in relation to the materials of the crucible for melting turned out to be an obstacle to the emergence of the new technology, which required the introduction of platinum crucibles. New lanthanum glasses often contained oxides of the transition metals yttrium, niobium and tantalum, zirconium and hafnium, tungsten, cadmium. Sometimes even these details noted separately for marketing purposes, and lanthanum glasses were given names to whole lines of photo lenses.

In the USSR, unlike Japan, USA and both Germanium, thorium glasses were not mass-produced. The first lanthanum glasses, the so-called super-heavy crowns (SHC), were apparently obtained in the second half of the 1950s. The famous Soviet optician D.S. Volosov In “Photographic Optics” (1970, pp. 502-507) he mentions such materials as STK1 717.470 and STK6 747.509.

According to D.S. Volosov [“Photographic Optics”, 1970, p. 506] and the 1963 GOI catalogue (compiled by E.B. Lishnevskaya), STK6 glass was used in 1958 to calculate the new 50/2.8 lens by V. Sokolova and G.G. Slyusarev (also known for his work on aerial photography optics and the BTA telescope) – Industar-61.

The new lens featured significantly better correction of both spherical aberrations and astigmatism. Industar-61 turned out to be so successful that it was produced for decades, even reaching into the 21st century... However, STK6 glass was never in serial production, is not listed and does not even have exact analogues in the catalogs of materials GOST, IPZ or LZOS. The question arises: what kind of lens were they producing all these years?

The complex and interesting history of the production of the Industar-61 lens was established thanks to information from the archives of GOI and LZOS, as well as an analysis of serial samples of the lenses.

So, it turned out that the non-serial STK6 glass was quickly replaced by the STK9 743.503, which had similar parameters, which, of course, required some recalculation of the lens, apparently also made by GOI. The optical scheme of the new version of the lens was available to me, which made it possible to evaluate its optical properties. But due to the lack of information about the optical design of the very first version of the GOI I-61, the changes cannot be tracked.

This version of the lens is undoubtedly the best in comparison with the Industar-26m: the contrast level and resolution are significantly higher, the primary spherical aberration, field curvature and astigmatism are better corrected. The only difference is that the distortion has increased from -0.25% to -2.5% ("barrel"). What is surprising is the high level of uncompensated secondary spherical aberrations, which I have not observed in any of the serial Industar-61 lenses - be it the early Industar-61L or late Industar-61 LZ MSIt is very likely that neither the first nor this version of the Industar-61 lens were ever mass-produced.

In fact, there were only two versions of the Industar-61 lens that were in serial production. Thus, by analyzing the documentation, it was possible to establish that the latest version of the Industar-61 lens LZ MS used STK19-F4-F6-STK19 glasses as optical materials. STK19 744.504 glass has a very characteristic signature in the X-ray fluorescence spectrum with peaks of lanthanum, barium, zirconium, yttrium, tantalum and hafnium, but lacking cadmium peaks.

Meanwhile, all Industar-61 lenses from 1964 to 1986, as it turned out, have STK9 743.503 glass as the material of positive lenses, the characteristic signature of which is peaks of lanthanum, zinc and cadmium in the absence of peaks of tantalum, hafnium, yttrium, barium.

Therefore, all Industar-61 lenses produced in the USSR were most likely made using the optical scheme using STK9-F4-F6-STK9 materials. The optical scheme of this version was recreated using the optical scheme of the late and early lenses.

In this version, despite the increased field curvature due to the replacement of the light LF9 flint with a regular F6, mutual compensation of spherical aberrations allowed to significantly increase the image contrast at an open aperture in comparison with the GOI version. And if everything is so good, then why was the lens recalculated for the STK19 744.504 glass? And why was it necessary to produce this glass if there is already STK9 with similar parameters?

The answer is simple: the production of glass with a large amount of cadmium in its composition is dangerous both for the workers at the plant and for the environment. STK19 glass contains more expensive and rare, but much less toxic elements. Apparently, it is precisely because of the high cost of STK19, the recipe for the melting of which was developed back in the 1970s, that the melting of STK9 continues to this day - this material is still present in the current LZOS catalog.

Meanwhile, STK19 as a material for making positive lenses due to its dispersion properties turns out to be somewhat better than STK9. Thus, using STK19 instead of STK9 in the Industar-61 lens allows reducing the length of the secondary spectrum by 15% and significantly improving the optical quality of the lens.

Thus, there were at least four different calculations of the optical scheme of the Industar-61 lens, of which the two earliest were never mass-produced. The third version of the scheme became the main one and most of the produced lenses correspond to it. The latest versions of the lens - probably the Industar-61 LZ MS lenses produced after the collapse of the USSR - belong to the last, fourth version of the scheme, which should demonstrate slightly better optical properties.

Optical properties of FED Industar-61: comparison with Industar-26m and Industar-61 LZ MS

FED's Industar-61 lenses have a controversial reputation, especially when compared to Industar-61 LZ. In my experience, the quality variation of the late I-61s is quite serious – in some cases the lenses I-61 L/D turned out to be no better in comparison with their predecessor – I-26m. This is where the interest in comparing the early Industar-61 FED lens with the same Industar-26m arises: after all, in a similar test the early LZOS Industar-61L turned out to be significantly better in optical quality.

What is new in this test is that the optical quality was also assessed outside the 36x24mm field, which is valuable for owners of medium format cameras like the Fujifilm GFX with a 44x33mm matrix format. The camera used was Sony A7s с shift adapter with a shift along the long side of the matrix by 9 mm, which allowed fixing the lens field up to y'=29.5 mm with the required y'=27 mm. The test was performed with apertures from F/2.8 to F/8, with each change in aperture refocusing was performed on the central area.

Crops of the obtained images are shown below.

As you can see, the I-61 lens wins in image quality in the center of the lens field at F/2.8-F/4, and in the field – at any of the selected apertures. The test result can be called predictable, but only if the I-61 lens has a high-quality assembly, it can outperform the well-assembled I-26m in sharpness.

The I-61 and I-26m lenses differ significantly in their pattern due to the different balance of field aberrations. A detailed comparison of the lens images has already been made earlier, but we will still provide a few more illustrations. Below are examples of photos taken with the I-61 FED at F/2.8, including a panorama using shift adapter.

Then – paired shots on the I-26m.

I always liked the Industar-26m design more, although the low optical quality definitely limits its capabilities, especially on modern high-resolution matrices. The I-26m lens is very far from "ideal" and its design is difficult to reproduce in calculations, however, I managed to calculate a lens with 50/2.1 (!) parameters with optical properties and a design similar to the I-26m using modern materials while maintaining the Tessar design.

The following comparison – with the Industar-61 LZ MS 1986 – serves not only as an aid in choosing a vintage Tessar for a modern camera, but also as a confirmation of the previously put forward theses on the development of the optical design of the lens. This test was conducted in a similar manner.

Crops of the photos taken are shown below.

The observed difference between the images taken with this pair of lenses is very small and is most likely related to focusing accuracy (there is no single correct option for focusing a lens with significant residual spherical aberrations and field curvature). The most striking difference in the paired photographs is the color balance: the I-61 FED photographs have a yellow-green tint.

There is no noticeable difference between these lenses in the nature of the bokeh. Except for those cases when the aperture is closed, of course – in this case, the Industar-61 LZ MS gives itself away with six-pointed stars. Below is a photo on the I-61 FED, F/2.8-F/5.6.

Then – paired images on Industar-61 LZ MS.

The multilayer anti-reflective coating of the lenses and the best light protection of the Industar-61 LZ MS allow achieving better image contrast in comparison with the I-61 FED. Apparently, the transition to the new coating was extremely useful for this lens.

Comparison of the transmission spectra of the I-61 FED, I-26m and Industar-61 LZ MS lenses confirms that the coating in the I-61 and I-26m under consideration is the same. Most likely, this is a single-layer chemical enlightenment, but selected from the desire to neutralize the transmission spectrum, which is a big plus. otherwise color distortion may be too strong.

The reduced transmission in the short-wave region of I-61 FED may be due to absorption by glass (the yellowness of the glass is due to iron impurity). Industar-61 LZ MS has significantly better light transmission in comparison with I-61 FED, especially in the short-wave region. However, due to the difficulties with the normalization of spectra, quantitative assessment is difficult.

User experience. Photo examples

Although this lens is often called the "eagle eye" along with the original "Tessar" 50/2.8, I think this is more out of habit. If the Industar-61 was an "eagle eye" - then only at the time of its release, and in comparison with the Industar-26m. It is difficult to call this lens very sharp by modern standards: the I-61 has a fairly high level spherochromatic aberrations at an open aperture, which makes the image soft at F/2.8, and a very extended secondary spectrum, limiting the resolution at F/5.6-F/8, especially in macro photography. If we are to look for an “eagle eye” among Soviet optics, then, for example, this title can be claimed by Vega-9 50 / 2.1.

An important advantage of the I-61 in comparison with, for example, the Helios-44 (and many other simple lenses of the "double Gauss" type) is a more uniform distribution of sharpness across the field, without strong dips. Stopping down to ~F/5.6 allows for a significant improvement in image quality both in the center and across the field, including outside the 36x24 mm frame. The Industar-61 also has a low level of vignetting. All this makes the Industar-61 interesting for use with cameras with a 44x33 mm matrix, for which it EGF will be equal to 40 mm, i.e. the lens will be like a typical “pancake”.

The overall contrast of the image formed by the I-61 FED under normal conditions is good, at the level of other lenses with single-layer optics coating. Under difficult lighting conditions, the I-61 loses contrast due to color veiling caused by light scattering on refractive surfaces.

The Industar-61 has a characteristic "Tessar" bokeh, which is similar to the bokeh of other lenses with a similar optical scheme. It is probably not the same fashionable, like bokeh Helios-40, but it is not without its own zest, especially when compared with modern lenses. Despite the modest parameters, on a 44×33 format camera, pronounced bokeh can be obtained even with a full-length portrait, if you select the background.

I enjoyed using this lens as well as Industar-26m previously, for shooting using shift adapter on a full frame camera Sony A7s. Industar-61 in this capacity is much better suited for landscape photography due to its higher resolution, but it is also still good for portrait photography.

Below are some sample photos taken with the Sony A7s using a tilt-shift adapter.

Then - without a shift adapter on the same camera.

Conclusions

The Industar-61 lens, as it turned out, not only had many mechanical design options, but also optically different versions, of which only the last two, very similar to each other, were mass-produced. A well-assembled Industar-61 lives up to its calling - to replace the old Industar-26m and allow taking high-quality photos in the USSR of the 1970s relatively cheaply. Nevertheless, in today's reality, common opinions about some extraordinary sharpness of the lens are far from the truth, real scenarios for using this very affordable and simple lens with modern high-resolution matrices are not scenarios for the "eagle eye", but rather the opposite.