Review of the RO2-2M 75/2 film lens and test comparison with OKS1-75-1

Material on the RO2-2M 75/2 lens specifically for Radozhiva prepared Rodion Eshmakov.

The OKS1-75-1 75/2 lens for the test (see below) was provided by Alexander Korolev.

RO2-2M 75/2 is an old film lens produced in the USSR at the Krasnogorsk plant in the mid-2th century. There was also an older RO2-2 lens produced in Leningrad, which differed from the later RO2-2M in its antireflective coating and, probably, in the brands of optical glass. The PO2-XNUMXM is believed to be the predecessor of the lens OKS1-75-1 (Leningrad), although, most likely, both lenses were produced approximately simultaneously for a long time, before they were both replaced by OKS6-75-1 75/2.

Like OKS lenses, film optics of the RO series are highly valued in the secondary market - partly due to attractive parameters and characteristic images, partly due to the prevalence of the myth about the superior quality of Soviet film optics in comparison with photographic and projection ones.

There is also a short-focus version of the RO2-2M 75/2 lens - an interesting RO3-3M 50/2 lens https://radojuva.com/2020/06/ro3-3m-2-50-ussr-lens-review/.

Technical characteristics (catalog of lenses by A.F. Yakovlev, vol. 1, pp. 264-266, GOI ONTI, 1970):

Optical design – 6 lenses in 4 groups, “Planar” / “Helios”;

Focal length - 75 mm;
Relative aperture - 1: 2;
Estimated frame format - 16×22 mm, covered - 44×33 mm;
The back focal segment is 48,9 mm;
Light transmission coefficient (not less) – 0,81.

Design and adaptation

The lens is usually found either in the form of a separate lens unit with a diaphragm, or in a factory frame with a focusing mechanism. You can also find various converted versions of PO2-2M.

I received the lens in the form of a lens block without a diaphragm, and the blades and crown of the iris diaphragm were removed from the lens, and the drive mechanism was left untouched. The problem was quickly solved by installing a ten-blade diaphragm with a light diameter of 29 mm (a nine-blade diaphragm with a diameter of 28 mm, found in Industar-55U, Industar-58U) right on top of the remains of his own. The aperture scale markings have been re-engraved.

The standard PO2-2M focusing mechanism is usually not very convenient for converting a lens for modern cameras, so for this they often use Helios-44m or other similar lens bodies. This adaptation option has a drawback - the minimum focusing distance is too long. My lens block was installed in a Chinese macrohelicoid M52-M42 25-55 using a plastic sleeve made by 3D printing. With this adaptation option, it is easy to ensure compatibility of the lens with SLR cameras. The adapted PO2-2M is distinguished by its small dimensions - comparable to Jupiter-9 85/2.

The RO2-2M optics are single-layer coated. The color of the flare of the coating may differ for lenses from different years of production, which is very typical for Soviet optics. The lenses on my lens have a pinkish-purple and peach tint, but when held up to light the lens looks greenish-yellow. There are also versions with a pink and yellow finish.

Photos of the adapted lens are shown below.

Optical design

Structurally, the RO2-2M lens is very close to Helios type lenses. It differs from the Helios-44 type lens in the brands of optical glass. Most likely, both PO2-2M and OKS1-75-1 use heavy and ordinary flints in negative lenses, but “old” heavy crowns and barium flints in positive lenses - like the Helios lens described in the GOI lens catalog (1963). 2 75/2. At the same time, it is known for sure that Helios-44 uses “new” heavy crowns and barium flints in positive lenses and light flints in negative ones. At the same time, the logic for choosing glass grades in both Helios-44 and lenses like Helios-2 75/2 (the probable prototype of RO2-2M and OKS1-75-1) is very similar.

Analysis X-ray fluorescence method (Bruker M1 Mistral) of the front and rear lenses of the lens shows that the front lens contains significant amounts of barium (Ba), zinc (Zn) and antimony (Sb), but does not contain lead - therefore, the front lens is made of TK type glass, and with a relatively high dispersion, which is similar to glasses of the TK4-TK9 line. In the spectrum of the rear lens of the objective, signals of barium (Ba), zinc (Zn), lead (Pb) and arsenic (As) are recorded, which corresponds to BF type glass. Below are images of the recorded spectra.

Thus, the RO2-2M lens is really close to Helios-type lenses in terms of the choice of optical glass brands.

Comparison of image quality and bokeh of RO2-2M and OKS1-75-1

There was no reliable information about the differences between the RO2-2M and OKS1-75-1 in the sources known to me, and questions about the need for the coexistence of two almost identical lenses arise naturally. To understand the nuances of the optical characteristics of both lenses, I conducted paired testing of image quality under equal conditions when focusing at infinity with apertures from F/2 to F/8. Camera used – Sony A7s.

Below are photographs taken on RO2-2M.

Then - photo on OKS1-75-1.

It is easy to see that due to different coatings, lenses have different color rendition. The violet coating of OKS1-75-1 moves the color more strongly into the yellow region. However, this lens had no less different anti-reflective coating options than the PO2-2M.

Let's look at 100% crops from the resulting images.

Under the indicated testing conditions on apertures F/2-F/2.8, the PO2-2M and OKS1-75-1 lenses behave equally well in the central area of ​​the frame, but at the edge of the frame the sharpness of the OKS1-75-1 looks much higher than that of the PO2- 2M. At apertures F/4-F/8, the lenses behave almost indistinguishably.

Thus, the difference between the RO2-2M and OKS1-75-1 lenses lies in the method of correcting field aberrations: OKS1-75-1 has a pronounced spherical aberration of inclined beams, introducing a soft effect that intensifies from the center to the edge, preserving image details, while while PO2-2M appears to have a coma as a residual aberration. In other words, the OKS1-75-1 lens is corrected similarly to lenses like Helios-40 or Helios-44, and RO2-2M should be different from them.

To illustrate this fact, I prepared paired photographs on RO2-2M and OKS1-75-1 such that the bokeh of the lenses was clearly visible.

Below are photographs taken on RO2-2M.

Then - photo on OKS1-75-1.

As you can see, the PO2-2M lens produces bokeh that is more reminiscent of “scales,” while OKS1-75-1 is more reminiscent of Helios-44 or Helios-40.

The “scales” in the bokeh of the PO2-2M are formed due to the influence of the coma, which redistributes the energy in the disc/lemon of the bokeh along the edge so that the border facing the center of the frame becomes much brighter than the border facing away from the center of the frame - and the further away from the center, the stronger the effect becomes.

In OKS1-75-1, when moving from the center of the frame to the edge, the brightness of both sides of the disk/lemon bokeh increases, which is due to the influence of spherical aberration of inclined beams.

This example perfectly shows the importance of human participation in lens development: it turns out that local optima of an optical design can be so similar and close that only a person can ultimately select the right one in accordance with existing requirements. In other words, lenses can have a very, very similar design, but different optical quality and design, and the achievement of any specific result in this case depends entirely on who is considering the lens.

Optical properties and user experience

The PO2-2M lens has good sharpness with an open aperture in the central area of ​​the frame 36x24 mm. At F/2.8, residual aberrations in the center of the frame disappear, resolution is limited by longitudinal chromatism, and further aperture makes sense only to correct field distortions and increase depth of field. The disadvantage of the lens is the pronounced drop in sharpness inherent in planar optics from the center to the edge of the frame - outside the APS-C frame in the aperture range F/2–F/2.8 the lens behaves mediocrely. Field curvature and astigmatism in this lens are corrected well (which cannot be said about many projection lenses), therefore at F/5.6 it is already possible to achieve image quality sufficient for landscape photography.

The lens can be used with 44x33mm medium format cameras (Fujifilm GFX) or tilt-shift adapter on full-frame cameras, but geometric vignetting looks large already in the 36x24 format and amounts to at least 50%.

Under normal lighting conditions, the PO2-2M has good image contrast, no worse than other similar lenses with a single layer of coating. In backlight it easily catches colored highlights, rainbows and other artifacts. The color rendition of the lens is shifted to the yellow-green region, and sometimes it is not easy to cope with the peculiarities of the transmission spectrum (see below) and scattering when processing photographs. This problem is inherent in many lenses with single-layer (or low-quality multi-layer) coated optics.

The bokeh of the RO2-2M differs from the background blur of various types of Helios and looks interesting in itself.

The following are examples of photographs taken on the PO2-2M and a full-frame camera Sony A7s, made as “shiftorama” using shift adapter.

Then - photos taken in the usual way using a PO2-2M and a Sony A7s camera.

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

The old film camera RO2-2M 75/2 is a rather convenient and pleasant lens, which, however, is inferior in optical quality even to OKS1-75-1. A big plus in favor of the RO2-2M is its image, which is different from lenses like “Helios”, because this lens is not exactly “Helios-40, but darker and lighter,” but some more original solution. Typically, the cost of PO2-2M lenses is high, so do not forget about more affordable alternatives, also endowed with interesting designs: Jupiter-9 KMZ 85/2, film projection 35KP-1,8 / 75, Zh-53 75/2 и PO500-1 90/2.

You will find more reviews from readers of Radozhiva here и here.