MP RSFSR GLAVOCHTEKHPROM PLANT #6 ★ F=7.7cm ★ - a review of the legendary lens adapted for modern cameras + computational bonus

Material on the lens MP RSFSR GLAVUCHTEKHPROM PLANT No. 6 ★ F = 7.7cm ★ especially for Radozhiva prepared Rodion Eshmakov.

This lens with a mega-strange unpronounceable name (I will continue to call it simply “Glavuchtekhprom”) is a projection lens from the FGK-49 filmoscope, which was produced in Zagorsk (now Sergiev Posad) at School Instrumentation Plant No. 6. In essence, this is the cheapest, simplest and low-quality projection lens that Soviet projection installations were equipped with. In Radozhiv, Glavuchtekhprom has already appeared in semi-joking review, and in this article the capabilities of the lens are evaluated, although also not without irony, but in the most serious way. And even closer to the end of the article, there is again a “computational bonus”.

Lens specifications:

Optical design - 2 lenses in 2 groups, a symmetrical lens of the "Periscope" type;

Focal length - 77 mm;
Relative aperture - 1:3 (factory), 1:2 (actual, when replacing the diaphragm);
Covered frame - up to 60×60 mm;
Filters thread - absent;
Case material – plastic;
Features - a projection lens, does not have a variable aperture, mount to the camera and an acceptable focusing mechanism.

Design and adaptation

Glavuchtekhprom is ahead of modern lenses in many ways. For example, already in the 50s, its body was completely made of polymer materials, including interlens rings - just like Samyang today, and the dark glossy exterior of the lens unit resembles Sigma Contemporary lenses.

The downside of such an original lens design is perhaps the lack of the ability to focus and change the aperture. Therefore, the objective lenses made using high-precision technologies (traces of a precision tool on their ends are up to 0.5 mm in size) were removed from the plastic case, for which it was only necessary to unscrew the plastic rear slotted nut.

Unfortunately, the lens is rarely found on the secondary market in good condition - apparently, all successful specimens have long been sold out by Chinese collectors. I received as many as three lenses, and it turned out that in two of them one of the lenses was handicraftly replaced with a piece of plastic, and there was only one glass lens of decent preservation, but with a Soviet anti-reflection coating - the most nanocrystalline in the world. In the third lens, both lenses were good, but did not have enlightenment. An interesting fact: all glass lenses were of different thicknesses, but visually did not differ in curvature. I picked up two close in thickness to each other - one with enlightenment, the other without.

Especially for the adaptation of the lens, a new metal case was made to order (there was a rollback in technology), and an iris multi-blade diaphragm was selected, which was located in the middle between the lenses due to the symmetry of the lens. The Chinese macrohelicoid M52-M42 17-31 was selected for focusing. Below are photos of the resulting lens.

In the new look, Glavuchtekhprom has turned into a super-compact 77mm f/2 portrait lens with M42x45.5 mount, compatible with all modern cameras – a worthy alternative to the Canon RF 85mm F/2 Macro!

Optical properties

On an open F / 2 aperture, Glavuchtekhprom draws incomprehensible foggy perspectives under the influence of the strongest spherical and chromatic aberrations. When aperture is up to F/2.8, the lens loses its mark (and spherochromatism) a bit, and the Japanese FUJINON.SF 85mm F/4, which for some reason is considered a reference soft lens, is already catching up with it (they just mixed up two long names - RSFSR MP GLAVOCHTEKHPROM PLANT No. 6 ★ F=7.7cm ★ and Fuji Photo Film EBC FUJINON.SF 85mm F/4), although the Soviet lens is still much more “mysterious” around the edges due to astigmatism and field curvature. With F / 4-F / 5.6, the lens acquires a sharpness that can already be discussed without jokes, and with F / 8 it is even surprisingly sharp for such a primitive optical scheme - and only residual chromaticity and curvature of the field give out "Periscope" in it.
The overall contrast of the image is highly dependent on the set aperture - from about F / 2.8-F / 4 it turns out to be at a very good level. Many multi-lens lenses have worse image contrast than the adapted Glavuchtekhprom.

The color rendition of the lens is practically without distortion - due to the fact that the window glass from which it is made (actually it is some kind of K8) does not contain lead additives, and there is only one lens in the lens with anti-reflective coating and it does not greatly affect the color. affects.

The bokeh of the lens depends on the selected aperture value. In general, Glavuchtekhprom has a soft background blur, since the bokeh discs do not have a clear border. Very often, however, blue edging (spherochromatism) can be observed. The foreground of the lens, on the contrary, blurs in a lurid manner, similar to a very bad Planar. The prefocal bokeh discs have bright orange edging.

A nice feature of the lens is the almost complete absence of distortion.

In fact, Glavuchtekhprom has quite sufficient optical quality, if you don’t try to shoot everything with it on an open aperture. In some cases, at F / 5.6, this lens is able to give a picture that is much more interesting and pleasant than that of “normal” lenses like 75/2. And, of course, in the F / 2-F / 4 range, the lens is very interesting in black and white photography. The Soviet "periscope" definitely managed to pleasantly surprise me.

The following are sample shots taken with the Sony A7s full-frame camera. Part of the photo (without EXIF) taken as "shiftorams".

I also offer some photos in black and white.

Computational bonus: calculation and comparison of "monocle" and "periscope"

After looking at the photos, many will think to themselves something like “well, well, this is very similar to monocle! And in some ways they will be right, but in some they will not. What exactly gives another lens to the "periscope" that the "monocle" does not have?

Let's take a closer look at the capabilities of a single lens lens. It has 5 parameters in total:

• Glass properties (refractive index and dispersion);
• Two radii of curvature;
• Lens thickness;
• Aperture diaphragm position.

In order to get a good single lens lens, the glass must have as little dispersion as possible and as high a refractive index as possible. The downside is that these two properties are inversely correlated - as you can see in the figure below, the higher the refractive index n, the smaller the Abbe number v (i.e., the greater the dispersion). Taking into account the fact that glasses of the FK and TFK type are expensive and unstable in humid air (and LZOS does not have them, but TFK11 once produced IPZ), ordinary K crowns are the optimal choice of glass, for example, the cheapest and most common borosilicate glass K8 , which is in every catalog under some name of its own (BK7 in the Schott catalog, for example).

Since only one optical material is used, chromatic distortion is uncorrectable - the magnitude of the secondary spectrum depends only on the dispersion of the glass.

By the way, it follows from this that "a monocle from Helios" is a very dubious lens: the front lens of Helios lenses (and many other Soviet lenses) is made of TK16 glass, which, although it has a rather high refractive index (~ 1.62), also has too much dispersion.

The radii of curvature have the strongest effect on the balance of monochromatic distortion. The number of parameters for a single lens is not enough to deal with all aberrations at the same time. Therefore, it is only possible to achieve sufficient correction of any of the five Seidel aberrations (spherical aberration, coma, astigmatism, field curvature, distortion), while the rest can only be influenced by aperture. It is important to note that the curvature of the field cannot be corrected in principle in lenses consisting only of positive lenses.

If you want to get the simplest single-lens telescope, then the parameter to be minimized will be spherical aberration, which affects the image quality at small angles of view (for telescopes, it is usually <<5°). The condition of minimum spherical aberration is satisfied by lenses with approximately plano-convex (for n~1.5-1.7) or meniscus (for n>1.7) shape.

In photography, however, lenses with large field of view angles (20–80°) are more commonly used. Therefore, it is much more important to pay attention to field aberrations (coma, astigmatism). To correct them, the lens needs to be given a meniscus shape. It becomes clear that here, too, the “monocle from Helios” is in flight.

To correct the coma, a variation in the position of the aperture diaphragm is also used, which will determine which part of the lens the oblique beams of light will pass through - located far from the axis or close to it. Interestingly, it is the pre-lens aperture arrangement that gives the best image quality in the case of a "monocle". Despite this, most single-lens photographic lenses in production had an aperture behind the lens, for both aesthetic and reliability reasons.

The thickness of the lens affects the image quality rather weakly, therefore it is chosen for practical reasons. From above, the thickness is limited by the mass of the lens and the allowable glass consumption, and from below - by the thickness of the edge (should be more than 0.7-1.5 mm, depending on the diameter of the lens and the technological level of production).

I calculated in ANSYS Zemax 13 a lens from one lens (K8 glass) with a focal length of 75 mm, a relative aperture of F / 8 and a field of view angle of 30 ° (calculation for a frame of 36 × 24 mm).

The lens in terms of its optical quality at F / 8 does not even reach something like Triplet 78 / 2.8 wide open at F/2.8 due to poor correction of spherical aberration even at this relative aperture. The lens I calculated has well-balanced astigmatism and coma, distortion is about -1% (an imperceptible “barrel”) - but this is only 30 °.

When you try to increase the relative aperture of the “monocle”, the correction of spherical aberration will sharply worsen, astigmatism will grow, destroying sharpness at the edges of the frame - therefore, it is difficult to shoot with such a lens, even specially calculated, at apertures above F / 5.6.

If you try to increase the angle of the field of view, then the problem of balancing astigmatism and coma will make the lens bend even more, worsening the correction of spherical aberration. The aperture will have to be clamped to f / 16 and beyond to achieve more or less decent quality at an angle of 60 °. In this case, distortion can exceed ten percent, which is already very, very noticeable. In the XNUMXth century, such lenses were sometimes called "landscape lenses" and had a very, very small relative aperture - it was impossible to shoot anything with them except for static landscapes.

How does turning a "monocle" into a "periscope" help improve image quality? Consider the case of a completely symmetrical objective of two identical lenses.

It must be understood that adding a second lens does not solve the problem of chromatic aberration. To correct chromatism, one would have to add a negative lens, not a positive one, and made of glass with greater dispersion - and this would lead us to an achromat (Chevalier landscape lens), which has the same disadvantages in the form of field aberrations as monocle.

The key advantage of the "periscope" over the "monocle" and the Chevalier achromat is the symmetry of the lens design. Symmetry makes it possible to sharply improve the correction of field aberrations - coma and astigmatism. The curvature of the field still remains uncorrectable due to the absence of negative components in the system.

I calculated a symmetrical "periscope" of the Glavuchtekhprom type with parameters of 75 mm F / 5.6 30 ° using K8 glass.

As you can see, with twice the relative aperture of the lens, the performance turned out to be generally higher than that of the “monocle”. The coma in the lens is not completely corrected, which can be seen from the asymmetry of the spots - the fact is that a completely symmetrical system can only be free of coma when it works at a 1:1 scale.

If complete symmetry is abandoned, the “portrait” “periscope” can be significantly improved: a complete correction of the coma becomes possible. At the same time, the lens is already losing its charm (and manufacturability) a little, ceasing to be just a “double monocle”. And the rejection of symmetry leads to an increase in distortion. It is for this reason that historically the first ultra wide angle (>110°) Goerz Hypergon lenses were perfectly symmetrical periscopes of two nearly hemispherical lenses.

The asymmetric version of the "periscope" is very similar in the shape of the spots to the Petzval lens and is essentially its non-achromatic version with undercorrected spherical aberration. A more or less complete correction of chromatism while maintaining undercorrected spherical aberration by replacing one of the lenses with an achromatic doublet leads to a soft lens called "Verito".

Thus, even in a completely symmetrical case, the "periscope" has a significant advantage over the monocle in the working relative aperture and can be used at F / 3.5-F / 5.6, which was observed in practice when shooting at Glavuchtekhprom. For use with modern cameras, the "periscope" is a much more suitable lens than the "monocle": using the "periscope" you can achieve the desired effects such as "glow" contours or "haze" with a minimum presence of undesirable types of hard blurring of the edges of the image and the impossibility of obtaining a sharp off-axis images. Fairly high aperture "periscope" makes it possible to use it effectively in any lighting conditions.

All reviews of film projection and filming lenses:

1. RO3-3M 2/50
2. RO2-2M 75/2 (review from the reader)
3. PO 500-1 F9 CM. 1: 2 P (review from the reader)
4. LENKINAP RO500-1 F = 9cm 1: 2 P (review from the reader)
5. ЛЭТИ-60/60М F=92 1:2 (review from the reader)
6. 2/92
7. F = 92 1: 2
8. ОКП-6-70-1 F=70 1:1,8
9. LENKINAP OKS1A-75-1 F=75 1:2 P (review from the reader)
10. LOMO RO501-1 F = 100 1: 2 (+ materials from the reader)
11. LOMO RO500-1 F = 90 1: 2
12. 16KP-1,4 / 65 (review from the reader)
13. 35KP-1,8 / 65 (review from the reader)
14. 35KP-1,8 / 70
15. 35KP-1,8 / 75 (review from the reader)
16. 35KP-1,8 / 85
17. 35KP-1.8 / 100 (review from the reader)
18. 35KP-1.8 / 120 (review from the reader)
19. 35KP-1,8 / 120 (with aperture, reader's review)
20. LOMO P-5 F = 90 1: 2 (review from the reader)
21. LOMO P-5 F = 100 1: 2 (review from the reader)
22. LOMO OKS1-22-1 F = 22 1: 2.8 (review from the reader)
23. ЛОМО ОКС1-40-1 40/2.5 (review from the reader)
24. LOMO OKS1-300-1 F = 300 1: 3.5 (review from the reader)
25. LOMO OKS11-35-1 F = 35 1: 2 (review from the reader)
26. LOMO J-53 F = 75 1: 2 (review from the reader)
27. LOMO J-54 F = 85 1: 2 (review from the reader)
28. LOMO OKP4-80-1 F=80 1:1,8 (review from the reader)
29. LOMO OKP8-90-1 F=90 1:2 (review from the reader)
30. Tair-41 50/2 (review from the reader)
31. KO-120 1: 2,1 120mm
32. KO-90 1: 1,9 F = 9cm (review from the reader)
33. KO-120M 1: 1.8 F = 120mm (review from the reader)
34. KO-120M 120 / 1.8 with a diaphragm and helicoid (review from the reader)
35. KO-120 1: 2.1 F = 12cm (review from the reader)
36. GOZ “KO-140” 1:2,2 F–14cm (review from the reader)
37. MP RSFSR GLAVOCHTEKHPROM PLANT №6 ★ F=7.7cm ★ (review from the reader)
38. MSO USSR SSD UPP-1 ★ KHARKIV ★ F-7 CM ★
39. Schneider Super Cinelux 70/2
40. Meopta Meostigmat 100/1.7
41. Projection aplanates: "Petzvali" and "Richter"

The names of the lenses correspond to their exact spelling on the body.

Conclusions

"I'm shooting at the MP RSFSR GLAVUCHTEKHPROM PLANT #6 ★ F=7.7cm ★!" - sounds cool, because this lens is an excellent classic symmetrical periscope, manufacturability and unpretentiousness to the production conditions of which was once allowed to equip schools with filmscopes. The rough design of the lens is not a hindrance today: even with the adaptation of many better manufactured lenses sometimes you have to make the body from scratch. The redesigned lens demonstrates a truly unusual and interesting picture and leaves Helios crafts made by different "masters" far behind. For lovers of soft optics, I boldly recommend the “periscope”!

You will find more reviews from readers of Radozhiva here.