Material especially for Radozhiva prepared Rodion Eshmakov.
Adapted RO2-2M, Zh-53 and RO506-1.
When looking for a manual high-aperture portrait lens, you can often stumble upon adapted old film projection and film lenses. Moreover, around the old Soviet film optics there was almost a cult, similar to the cult Helios-40, while Soviet film projection lenses with the same parameters are treated with less reverence. This article clearly shows the difference in image quality and its nature between the RO2-2M 75/2 film lens https://radojuva.com/2023/11/ro2-2m-75-2/ and projection lenses of two different series - LOMO J-53 75/2 и PO506-1 80/2. For the first time, it is shown what exactly is the difference between RO and Zh projection lenses, which, as a rule, are considered identical up to the focal length.
Design differences
All lenses studied in the article are six-element lenses of the “Double Gauss” type (“Planar”, “Helios”...) with close focal lengths and the same aperture f/2, made at the same technological level: the lenses use ordinary heavy crowns, flints, barium and heavy flints, the optical design does not contain elements made of low-disperse or highly refractive glass, special flints - a classic, in a word. Therefore, the main differences between lenses must be related to the balance of different optical distortions.
The degree of aberration correction is greatly influenced by the ratio of the rear focal length to the focal length f'/F, which is slightly larger for the PO506-1 and Zh-53 projection lenses (0.68) in comparison with the PO2-2M film lenses (0.65). Increasing the f'/F ratio usually leads to compromises. For example, the relative total length of the optical system of L/F projection lenses is also increased in comparison with PO2-2M.
Table of selected characteristics of lenses RO2-2M, Zh-53 and RO506-1. Sources: catalog of lenses by A.F. Yakovleva, vol. 1-2, GOI ONTI, 1970
It is easy to notice that projection optics have large diameter lenses - apparently, vignetting when displaying a 16x21 mm film frame was considered unacceptable. On a 36x24 mm frame, the vignetting of projection lenses is noticeably lower than that of the film PO2-2M - the measured values for PO2-2M, Zh-53 and PO506-1 are 50%, 45% and 40%, respectively. Vignetting, that is, limiting the aperture ratio for off-axis light beams, has a significant effect on correcting field aberrations.
The large dimensions of the projection lenses themselves are associated with the use of the autocollimation method during assembly: each lens is placed in a metal frame and centered in the machine chuck when the metal is turned before assembly. The method allows, subject to technology, to achieve the most accurate positioning of lenses. The film PO2-2M uses a conventional bulk assembly, which requires greater precision in the production of the lenses themselves to obtain an assembly quality similar to autocollimation. I would venture to suggest that this is due to the fact that projection lenses experience thermal stress during operation: metal frames promote heat dissipation and a more uniform thermal expansion of optical elements.
The differences in the anti-reflective coating used on the lenses cannot be considered key between shooting and projection optics and rather depend on the manufacturer. OKS LOMO series lenses usually have coatings similar to RO and Zh projection lenses produced at the same plant.
It turns out that Soviet film projection lenses are more complex in the mechanical design of the lens unit in comparison with film lenses and are designed with more stringent requirements for the values of the rear focal length and vignetting.
Image quality
To compare the quality of the image formed by the PO2-2M, Zh-53 and PO506-1 lenses, a comparative test was carried out under equal conditions when focusing at infinity using a full-frame Sony A7s camera. White balance fixed: it is easy to notice differences in color rendition between lenses with different types of coating. Below are 100% crops of images taken at different aperture values - from F/2 to F/8.
At open aperture, all three lenses differ significantly from each other. PO2-2M is sharp in the center, but towards the edge of the frame the sharpness drops monotonically under the influence of spherical aberration of oblique beams and coma. Zh-53, unexpectedly, turns out to be the weakest optically, demonstrating a fairly high level of spherochromatic aberrations in the center and thirds of the frame, astigmatism and field curvature in the corners. The PO506-1 is the sharpest of the three lenses in the center of the frame and in the thirds, but in the corners it has image quality no better than the Zh-53. A similar situation is observed at the F/2.8 aperture.
At F/4 you can see how the Zh-53 is slightly better than the longer focal length PO506-1 in the corners of the frame, which is counterintuitive.
At F/5.6, the PO2-2M turns out to be better than the film projection ZH-53 and PO506-1 in everything. Only the Zh-8, but not the PO53-506, improves the quality in the corners to F/1.
It can be concluded that the PO2-2M 75/2 lens is corrected for a larger field than the 35 mm film frame, and is therefore best suited for full-frame and medium format cameras. At the same time, the PO506-1 turns out to be noticeably better in image quality in the central area of the frame, and therefore would be perfect for APS-C cameras. Zh-53, apparently, is some intermediate design option, located closer to RO506-1.
Character of the image: RO2-2M and Zh-53
The cinematographic RO2-2M and the projection Zh-53, with equal parameters of 75/2, have very different bokeh patterns. The RO2-2M in this respect resembles Helios type lenses and strongly twists the background, while the Zh-53 does not twist the background. Examples of photographs taken on the PO2-2M and full-frame Sony A7s are below. Some of the pictures were taken using a shift adapter as "shiftorams".
Due to pronounced astigmatism outside the 36×24 frame, the bokeh of the Zh-53 lens noticeably deteriorates and becomes “comatose”, like some old 50/1.4 lens, so it is not very suitable for medium format cameras like the Fujifilm GFX. Within the 36x24 frame, bokeh looks quite stylish, even “modern” in some ways. Below are examples of photos from LOMO Zh-53 in similar conditions.
Character of the image: Zh-53 and RO506-1
The differences in the drawing are much less obvious when considering the pair Zh-53 and RO506-1. There is an opinion that Zh and RO lenses do not differ at all in terms of bokeh, but this is not so. The higher level of higher spherical aberrations in the Zh-53 leads to the appearance of clear bright fringing on the bokeh disks, while this is unusual for the PO506-1. It is also noticeable that astigmatism has a stronger effect on the bokeh of the PO506-1 in the corners of the frame, which is especially noticeable when shooting "shifters".
Below are photos taken with LOMO Zh-53 and full-frame Sony A7s, some of the photos were taken using shift adapter.
Then - examples of photos on PO506-1 under the same conditions.
Conclusions
Film projection lenses of the RO and Zh series are not so much analogues of film projection lenses RO2-2M and OKS1-75-1, but rather alternatives. Short throw projectors have mediocre image quality in the corners of the 36x24 frame and poor image quality outside of it, which limits their use on 44x33 mm matrices. Moreover, PO projection optics apparently have better aberration correction than the Zh series optics: this is probably why the very common PO500-1 90/2 has gained popularity among users of APS-C and full-frame cameras. It remains unclear why two series of film projection lenses (Zh-5x and PO50x-1) were produced simultaneously in the USSR, if RO lenses have an advantage when used for their intended purpose. Write in the comments your thoughts on this matter, as well as thoughts about the bokeh of Soviet film and projection lenses.
You will find more reviews from readers of Radozhiva here и here.
Thanks, interesting article!
If the lens is initially designed as a projection lens, then its best characteristics should be realized at a short distance from the screen. Which is often similar to the distance for a portrait.
A cinema lens will give the best picture at infinity, which is what it is designed for by default.
Yeah, that's why within APS-C the PO506-1 is much better than the PO2-2M.
Moreover: what kind of “portrait” distance was there in Soviet cinemas from the projector to the screen? Typically this was at least ten meters; even filmoscopes were designed for a projection of 3 to 8 m, to say nothing of film projection devices. For optics with a phase range of ~100 mm, 10 m and infinity are the same thing from the point of view of correction.
It would be nice to test at 2 meters
At 2 meters there is no point. In the range up to 70 cm it’s a different matter. There, the astigmatism and crooked field of RO506-1 and Zh-53 show themselves more clearly. The same story applies to OKP4-80-1 and the similar OKS6-75-1.
2 meters, approximately the distance for a portrait, that’s why it’s interesting. And you can shoot macro with other more suitable lenses.
From the point of view of quality, there is practically no difference - whether 2 meters or infinity.