Review, analysis and comparative test of the adapted projection lens LOMO RO500-1 F=90 1:2

Material on the lens prepared especially for Radozhiva Rodion Eshmakov.

Adapted LOMO RO500-1 90/2.

RO500-1 90/2 is a Soviet cinema projection lens belonging to the RO50x-1 series, which includes 7 lenses with focal lengths from 80 to 130 mm in different versions. This lens is both common and accessible, easy to adapt, and optically good – in a word, among exotic lenses, this is one of the best-proven lenses.

RO500-1 was produced for a long time at several plants (LOMO, Lenkinap, Refinery), depending on the year and place of manufacture, some characteristics of the lens (color formula and light transmission - i.e. type of coating, body materials, lens frame design) differed. This article is devoted to the lens manufactured by LOMO (1979), an analysis of its optical design and a comparison of the lens with a similar projection lens Meopta Meostigmat 90/2.

Technical characteristics (catalog of lenses by A.F. Yakovlev, vol. 1, pp. 349-350, GOI ONTI, 1970):

Optical design – 6 lenses in 4 groups, “double Gauss”;

Drawing of the optical scheme.

Focal length - 90 mm;
Relative aperture - 1: 2;
Calculated field of view angle – 19°;
Estimated frame size: 18.2×23.2 mm, covered: 44×33 mm;
The back focal segment is 60.6 mm;
Light transmission coefficient - 0.82;
Landing case diameter - 62.5 mm;
Length - 92.8 mm;
Weight - 795 g;
Features – projection lens, does not have its own focusing mechanism, iris diaphragm and camera mount in the factory version.

Design and adaptation

This LOMO RO500-1 lens has an aluminum body and is no different in design from other projection lenses of the RO50x-1 and Zh-5x series. The lens is easy to disassemble: just unscrew the slotted nut from one side and remove 4 aluminum frames with lenses and one thick aluminum ring, which serves as an insert between the halves of the double Gauss. When adapting, the diaphragm is installed by fixing it in this insert, for which you can use a machined or 3D-printed sleeve. The light diameter of the diaphragm should be not less than 29 mm. To control the aperture, a slot must be made in the sleeve and the lens body. Chinese macrohelicoids of the type M65-M42 25-55 mm – when using them, it is easy to achieve compatibility with both old SLR cameras and medium format mirrorless cameras of 44×33 mm format. The appearance of the lens after adaptation is shown below.

As you can see, my copy has faded paint on the title ring, and the same could be seen before adaptation and on the frames with lenses. To combat reflections during adaptation, all shiny and faded surfaces were painted matte black again.

In addition, during the work on the lens, it was discovered that the frames with lenses fit into the housing too loosely, i.e. there is significant axial runout of the lenses. This assembly defect is often found in Soviet optics assembled using the autocollimation method. Particularly strong violations of the assembly technology lead to the strongest decentering in the optical system and a catastrophic decrease in optical quality, which was demonstrated in the review of the Izyum projection lens 35KP-1.8 / 65. To eliminate assembly errors (small, but noticeable to a trained eye), I also adjusted my LOMO RO500-1 by adding spacers made of self-adhesive foil and rotating the frames with lenses relative to each other. As a result, decentering in the optical system was almost completely eliminated, as evidenced by the symmetrization of visible aberration spots.

Analysis of the optical design of the lens

Despite the mass production of RO50x-1 objectives, their optical design in the context of the optical materials used remained unknown. For example, the 1963 GOI catalog (compiled by E.B. Lishnevskaya) lists a huge number of 6/4 "Helios" type objectives, but there is not a single RO50x-1 or Zh-5x projection objective among them. Meanwhile, information about the brands of optical glass in these objectives can be useful for performing a comparative analysis with other objectives, and in itself has historical value.

To clarify the brands of optical glass used in the LOMO RO500-1 90/2 objective, I recorded X-ray fluorescence spectra from each of the objective lenses using a Bruker M1 Mistral device. Then, comparison spectra were recorded from objective lenses with precisely known brands of optical glass.

Abbe diagram – parameters of optical glasses from the LZOS catalogue.

Thus, it was reliably shown that the first, fifth and sixth lenses of the RO500-1 90/2 objective are made of BF11 glass (catalog GOST/LZOS/IPZ, see the figure above), the second lens is made of TK20 glass. Determining lead flints is a more difficult task due to the similarity of their spectra, but it can be logically assumed that the third lens is made of light flint LF5, and the fourth is made of heavy flint TF1. By the way, absolutely the same brands of optical glass in the same order are used in the objectives of the Zh-5x series, in particular - LOMO J-54 85/2.

Drawing of the optical diagram of the lens indicating the brands and parameters of optical glass.

It is curious that in the GOI catalogue there was not a single lens with exactly the same combination of optical glass grades, but there were very similar ones, among which was the probable ancestor of the RO50x-1 lenses – the OF-68 (card 38.05b) 100/1.65 1938 film projection lens.

The card of the OF-68 100/1.65 lens – a probable ancestor of the RO500-1.

This lens, as you can see, also has the first, fifth and sixth lenses made of the same optical glass TK21 (in the catalog it is designated as TK11). The glass differs from BF11 by a higher refractive index and greater dispersion. The second lens of the OF-68 lens is made of BF11, while in the RO500-1 its TK20 material has a significantly lower dispersion with a similar index.

The OF-68 lens has a similar ratio of the rear segment to the focal length as the RO500-1, as well as similar lens shapes. Probably, this lens was subsequently recalculated for a lower aperture value, which allowed to move away from dispersion glass type TK21 while improving the optical quality as a whole.

The choice of optical materials in the RO500-1 lens indicates the designers' desire to reduce the range of materials used. It is worth noting that the selected materials are more typical of 6/4 lenses with a larger relative aperture than F/2. Most likely, this approach is due to the fact that the lens must provide high optical quality immediately at an open aperture, while photographic lenses are shaped to allow for aperture maneuvering - and therefore must provide the highest quality at a limited aperture.

Optical properties: comparison with Meopta Meostigmat 90/2

Czech projection lenses Meopta Meostigmat are highly valued by enthusiasts outside the Russian-speaking world and are considered good optics. Indeed, Meostigmat 90/2 It is difficult to call it frankly bad, in comparison with the RO500-1 90/2 it even has one obvious advantage in the form of smaller weight and dimensions. However, a direct comparison of the optical quality and pattern of these two lenses has not been published before.
The far-field sharpness test of the lenses was conducted by taking paired photographs with the camera. Sony A7s under equal conditions for each of the tested lenses at apertures of F/2 – F/8. After each shot, refocusing was performed on the central area. The scene is shown in the picture below.

Test shooting object.

From the photographs taken, 500×500 pixel crops were made from the central, edge zones and corner of the image. The crops of the photographs are presented below.

As you can see, Meostigmat is inferior to LOMO RO500-1 in image quality across the field by a lot – the Czech projection lens cannot be saved even by quite strong vignetting, which RO500-1 practically does not have. Even in the central area of the frame, RO500-1 is ahead of Meostigmat up to F/4.

The pattern of these two lenses also differs significantly. Thus, the bokeh of the LOMO RO500-1 is practically free of twisting due to the absence of vignetting, but the background defocus disks have a noticeable bright edging. On the contrary, the bokeh disks of the Meostigmat 90/2 turn into "lemons" closer to the edges of the frame (especially on the medium format 44x33 mm), and the disks themselves do not have a bright edging in the central area of the frame. The pictures on the LOMO RO500-1 are presented below.

Then – paired photographs on Meostigmat 90/2.

Among the advantages of the RO500-1 over the Meostigmat is better image contrast, but the Czech lens has better color rendering, in my opinion: the LOMO RO500-1 has a single-layer anti-reflective coating of the lenses (probably physical, like MgF2) with a pale blue glare. This coating leads to reduced light transmission in the blue region of the spectrum, which is why the lens "warms" the picture.

Light transmission spectrum of the LOMO RO500-1 lens.

User experience

LOMO RO500-1 is well suited for use with full-frame cameras: the lens is capable of providing an acceptable level of quality both at open aperture and at limited aperture. Thus, at F/2, the lens sharpness is limited by spherochromatic aberrations in the center, spherical aberrations across the field and astigmatism in the corners of the frame. Outside the 36×24 frame, the level of astigmatism and field curvature increases, but to a much lesser extent than, for example, in a projection lens. LOMO J-53 75/2. The lens is capable of providing sufficient image quality at F/2 for portrait photography with a composition other than the central one, which distinguishes it from, for example, Helios-40 or OKS1-75-1.

At aperture values of F/5.6-F/8, the lens is sharp across the entire 36×24 frame and moderately sharp outside of it on a 44×33 format, i.e. it is also suitable for landscape photography.

The image contrast is good under normal conditions, but in backlight the lens often catches flare in the form of a whitish veil, and in side light bright lenticular rainbow highlights can appear across the entire frame - good special effects.

LOMO RO500-1 blurs the background well thanks to the combination of a focal length of 90 mm and a high aperture of F/2, has a pleasant "oily" bokeh - especially at waist-length portrait distances. With a frame of 36x24, the bokeh does not deteriorate towards the corners of the frame, on a medium format of 44x33, the blur somewhat loses its uniformity, like old lenses like 50/1.4 under the influence of astigmatism, which becomes noticeable when shooting a full-length portrait.

Low vignetting and high aberration correction allow the lens to be used without any particular difficulty on 44x33 cameras or on full-frame mirrorless cameras with tilt-shift adapter. Photos on Sony A7s и shift adapter are presented below.

Then – photos on a full-frame camera Sony A7s without using a shift adapter.

Conclusions

LOMO RO500-1 90/2 is a very pleasant lens in terms of its overall qualities, available to anyone who is not afraid to adapt it. As well as a similar lens in terms of parameters LOMO J-54 85/2, this lens can confidently compete with more expensive alternatives among vintage optics, be it OKS cinema lenses or European projection lenses.

All reviews of film projection and filming lenses:

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

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