Detailed review of the adapted film projection lens of the "Era" type 35KP-1,8 / 65 (IPZ, Izyum)

Material on the lens 35KP-1,8 / 65 especially for Radozhiva prepared Rodion Eshmakov.

Adapted 35KP-1,8/65. increase.

The 35KP-1,8 / 65 lens was produced in a small series at the Izyum Instrument-Making Plant (IPZ, Izyum, Ukraine) in the 1970-1980s and was intended for projecting movies shot on 35 mm film (a format close to APS-C ). Probably, this lens could also be produced by LOMO, but I did not come across such options. 35KP-1,8 / 65 can also be found under the name OKP2-65-1. This article is devoted to the adapted lens 35P-1,8/65 and also considers issues related to modeling the effect of decentering on the optical properties of the lens, adjusting lenses of the 35KP-1,8/65 type.

List of film projection optics produced by IPZ:

16KP-1,2 / 35 (OKP1-35-1), RO-110-1 35 mm F / 1.2 (six-lens scheme 6/5);
16KP-1,2/35 35 mm F/1.2 (8/5 eight-lens design);
16KP-1,2/50 (OKP1-50-1), RO-109-1А 50 mm F/1.2 (six-lens scheme 6/5);
OKP3-50-1 50 mm F/1.2 (8/5 eight-lens design);
16KP-1,4/65 (OKP1-65-1) 65 mm F/1.4;
35KP-1,8/65 (OKP2-65-1) 65 mm F/1.8 (Era 6/5 scheme, presented in this article);
35KP-1,8/70 (OKP6-70-1) 70 mm F/1.8 (scheme of the “Era” 6/5 type);
35KP-1,8/75 (OKP3-75-1) 75 mm F/1.8 (scheme of the “Era” 6/5 type).

Specifications 35KP-1,8 / 65 (according to the catalog of lenses Yakovlev A.F. 1970, v. 2, p. 225, "OKP2-65-1"):
Optical design - 6 lenses in 5 groups, "Era";

Schematic diagram of the lens 35KP-1,8 / 65.

Focal length - 65 mm;
Relative aperture - 1: 1.8;
Field of view angle - 23° (calculated), 45° (covered);
Frame format - 15.8×20.9 mm (calculated), 44×33 mm (covered);
Landing diameter - 52.5 mm;
The mass of the lens in the factory version is 340 g;

The dimensions of the lens are 35KP-1,8/65 in the factory version.

Features - a projection lens, does not have a focusing mechanism, attachment to the camera and an iris diaphragm.

Lens adaptation 35KP-1,8/65

The 35KP-1,8 / 65 lens has a design similar to many other Soviet projection lenses, such as LOMO J-54 85/2, PO500-1 90/2 or 35КП-1,8/75 75/1.8: its body is a tube that contains 5 aluminum "pancakes" with lenses, held by slotted nuts in front and behind. Between the second and third lenses there is a ring that sets the distance between the front and rear halves of the lens - in this place the aperture diaphragm of the lens is located.

To disassemble the 35KP-1,8 / 65, it is necessary to put different marks-risks on the lens body at the front and rear parts, then unscrew one of the slotted nuts and try to squeeze out a stack of pancakes with lenses from the opposite side of the body so that they do not rotate relative to each other . In this case, it is necessary to fix the position of the risks on the body relative to the stack of lenses and put appropriate marks on them. This step is necessary to reproduce the original (factory) assembly of the lens later, when the lenses are installed in the body according to the marks.

The 35KP-1,8/65 is a rare projection lens that uses a 52.5mm bore rather than 65.2mm or 82.5mm. Among the serial anastigmata, he is the only one at all, all the rest are aplanats KO series. The small diameter of the body facilitates the selection of the focusing mechanism and 35KP-1,8 / 65 can be installed both in the helicoids of defective lenses of the 135 / 2.8–200 / 3.5 type, and in the Chinese M58-M58 macrohelicoids. I used an M58-M58 25-55 helicoid with an M58-M42 shank to mount the lens on the camera. At the same time, the working length is more than enough to use 35KP-1,8 / 65 with SLR cameras. The difference between the diameters of the case and the helicoid was compensated for by a 3D printed sleeve.

For 35KP-1,8 / 65, as for a lens that is universal in terms of parameters, it is very useful to set the iris diaphragm. There is plenty of space for it between the second and third lenses in the lens. It is reasonable to use a diaphragm with a diameter of at least 26 mm, fixing it close to the third lens directly in the factory interlens bushing with screws. I used a 29mm ten-blade diaphragm. Very similar, but nine-bladed, can be removed from worn lenses Industar-55U и Industar-58U. I do not recommend spoiling lenses in good condition - they are interesting for shooting in themselves, although the I-55U without a diaphragm can become the basis for a home-made speed booster. The aperture ring was 3D printed for my lens.

Below are photos of the adapted lens.

35KP-1,8 / 65 turns out to be more similar in size to an 85/2 lens: it is larger than, for example, Jupiter-9 85/2. The reason is quite simple: projection lenses are designed so that their vignetting is minimal (which is why the calculated and covered frames differ so much), and the distance from the rear lens to the image turned out to be large enough so that the heat flux from the projector lamp does not lead to overheating and destruction of the frames. with lenses - and this leads to an increase in the dimensions of the circuit. But the lens converted in this way has a very small minimum focusing distance, which is a nice bonus for shooting a subject and near-macro.

Lenses "Era" D.S. Volosov. Establishing the types of optical glass in the lens 35KP-1,8 / 65

At first glance, the 35KP-1,8 / 65 lens uses an unremarkable optical design, very similar to the design of many other lenses, including the fast fifty dollars of old SLR cameras. In fact, this optical scheme is of considerable interest, since at a focal length (FR) of 65 mm, the lens has a back focal length (RF) of as much as 57 mm, that is, the ratio FR/FR = 0.88, which is very large for such a simple scheme. Compare for yourself: in the six-lens Helios-44 58/2, the rear segment is ~ 38.5 mm, and the focal length is 58 mm, that is, the ratio of the ZFO / FR = 0.66. Creating Lenses with Extended Back Segment - not an easy task, which usually requires the complication of the optical design - for example, the transition to retrofocus scheme.

The optical design of the 35KP-1,8 / 65 lens was actually developed by a Soviet optician David Samuilovich Volosov.

D.S. Volosov is one of the most famous and prolific Soviet opticians.

So, in 1966, a patent was registered for a photographic lens SU 182360 with parameters 50 / 1.8 and a similar 35KP-1,8 / 65 circuit diagram. Subsequently, this lens was also mentioned in the work of D.S. Volosov "Photographic Optics", was produced in small quantities under the name Era-1.

Patent SU 182360.

Comparison of drawings of optical schemes of lenses 35KP-1,8 / 65 (from the Yakovlev catalog) and the lens from the patent SU 182350.

Like the 35KP-1,8/65, the Era-1 50/1.8 lens (exact focal length 52.5 mm) had a large back segment: the ratio of ZFO/FR is 0.82. At the same time, to improve the optical quality in the Era-1 50/1.8 lens, it was supposed to use super-heavy (lanthanum) crowns, which are among the most expensive glasses. Subsequently, the optical scheme was developed in a rare lens desired by many collectors Era-6 50/1.5, which is the simplest optical design of the 50/1.4 type lens for SLR cameras!

GOST/LZOS/IPZ grades of optical glasses used in the Era-1 50/1.8 lens.

In the Soviet Union, however, the use of lanthanum glasses in mass production was very disliked. One can recall literally two lenses that were produced in a large series and used STK: this Industar-61 50 / 2.8 и Helios-81 50/2. Therefore, I could hardly believe that the large projection lenses 35KP-1,8 / 65 were made of lanthanum glass.

To determine the types of optical glasses used in the 35KP-1,8 / 65 lens, I used the X-ray fluorescence spectrometry method (XRFS device Bruker M1 Mistral), which allows you to determine the semi-quantitative elemental composition of the material without destroying it. In other words, to perform the analysis, it is sufficient to disassemble the lens and place each of the lenses in turn into the spectrometer. The results of the analysis are presented in the table below.

Thus, lanthanum and other rare earth elements were not detected in any objective lens. Consequently, 35KP-1,8/65 is made using grades of optical glass from the 30s and 40s. The main difference between BF and STK is a much higher dispersion of BF with a generally lower refractive index (BF16 - n=1.671, v=40.4; STK12 - n=1.692, v=55.0), which causes difficulties in correcting chromatic aberrations and forces the use of more dispersed TF flints.

Influence of lens decentering on the optical quality of the "Era" type lens. Elimination of assembly errors in the 35KP-1,8 / 65 lens

The biggest drawback, inherent in almost all lenses of the 35KP series produced at the Izyum Instrument-Making Plant, is the unbearably disgusting build quality. Most likely, if you buy some 35KP-1,8 / 70 even in an unopened box and put it on the camera, you will find that there is no optical quality at all, and only a connoisseur can shoot with such a lens helios monocle. Taking into account the fact that with lenses of the series 16KP such problems are not observed, it can be assumed that the production site for assembling the optics of 35KP lenses worked stably with a violation of technology throughout the entire period of its existence: after all, these lenses all use autocollimation assembly and, in general, should have close to ideal centering of optical elements!

Consider the effect of decentering of optical elements on image quality using the Era-1 50 / 1.8 lens as an example, by modeling in ANSYS Zemax 13. First, we determine the characteristics of an ideal lens - mainly the shape and size of aberration spots and frequency-contrast response (aka MTF / CFM / MTF) for the spectral function based on the spectral sensitivity of the sensor Sony A7R2.

Dimensions of aberration spots and MTF curves, chromatic aberration, astigmatic curvature of the field, distortion for the Era-1 50 / 1.8 lens with perfect assembly.

Now let's set a possible decentering from 0 mm to 0.1 mm for each optical element and simulate the lens with random assembly errors using the Monte Carlo method, fixing the change in the traced characteristics.

Sizes of aberration spots and MTF curves, chromatic aberration, astigmatic field curvature, distortion for the Era-1 50/1.8 lens when assembled with random centering errors up to 0.1 mm.

The study showed that the positioning error of the second and third lenses affects the image quality the most.

As can be seen, the presence of random decentering within the limits of only 0.1 mm leads to an increase in aberration spots in the central part of the image by one and a half to two times and the loss of symmetry of the axial spot. At the same time, the MTF sharply worsens in the central region of the image. Thus, as a simple visual criterion of assembly quality, one can first of all take the symmetry of the axial aberration spot, and then its size, which is related to the strength of the manifestation of the so-called “soft effect”.

Assuming the distribution of lens centering errors in the 35KP-1,8/65 lens to be random within a certain value, we can assume that by rotating each of the lens frames relative to the others, it is possible to find such a relative position of the lenses that the assembly errors will be partially compensated.

Thus, to align the 35KP-1,8/65 lens, you must first set the correct position of the second lens relative to the third, then the first relative to the second. Next, you can set the best position of the fifth and sixth lenses relative to the third and fourth in gluing. Naturally, each time the lens needs to be disassembled and reassembled, and to remember the positions, marks are required that can be applied with a marker. At the same time, in practice, it is most convenient to monitor the optical quality not by the symmetry of aberration spots, but by the shape and symmetry of the spots of near prefocals / outfocals: it is necessary to achieve their best symmetry and the smallest manifestation of a bright border and an outfocal spot.

Unfortunately, I did not take before-after photos, but by adjusting the two lens blocks of the 35KP-1,8 / 65 lens, I was able to achieve the same good quality, colossally superior to the original in both cases.

Optical properties 35KP-1,8/65

If, before the adjustment, none of the 4KP-5 / 35s that I had in my hands (1,8-65 pieces) showed reasonable image quality, then after the adjustment the lens became quite suitable for shooting. The lens at an open aperture demonstrates a rather sharp image both in the central region and across the frame 36 × 24 mm - in this it is ahead of, for example, an expensive filming lens RO2-2M 75/2. In the central region, sharpness is limited by spherochromatic aberrations (and, probably, by residual astigmatism arising from non-ideal assembly), and in the field, by higher-order aberrations, and, unlike helios-like lenses (ОКС1-75-1 75/2, Helios-44), in general, the level of coma and spherical aberration of oblique beams of the lens is quite low. Image quality is greatly reduced when using a lens in the macro range - you will need to stop the aperture to F / 2.8 at least to get images without excessive soft effect.

At closed apertures, the lens becomes very sharp both in the center of the frame and across the field, but at the same time, on a 36x24 mm frame, the pronounced influence of field curvature is already felt. Unlike the Era-1 50/1.8 lens, the 35KP-1,8/65 has a field that is curved away from the photographer at the edges. In addition, the 35KP-1,8/65 suffers from quite noticeable lateral chromatic aberrations, which appear as purple fringe on contrast contours at the edges of the frame.

The image contrast of the 35KP-1,8 / 65 is not high under normal conditions, but at an acceptable level - the lens uses a simple single-layer lens coating with a violet glare. Due to the fact that all lenses carry the same type of coating, the lens has a highly distorted light transmission spectrum, like Izyumsky 16KP-1,4 / 65.

Profile of the light transmission spectrum of the lens 35KP-1,8/65.

Violet highlighting increases transmission in the green region, but severely limits transmission in the blue-violet region. Because of this, under normal lighting conditions, the image has a rather simple magenta tone shift, which is easily compensated in the editor. However, if the shooting is carried out in backlight, then the violet veiling caused by the reflection and scattering of light at the glass-air boundaries is added to the shift in color reproduction due to distortion of light transmission. In this case, there is no longer a “correct” choice of image tone - it remains to choose between a green “main” tone with a “gray” veil and a “correct” “main” tone with a pronounced purple veil. The indicated effects can be used for artistic purposes.

The 35KP-1,8 / 65 lens has a fairly recognizable bokeh characteristic of Era lenses. Below are the simulated bokeh spots for the Era-1 50 / 1.8 lens, and you can also see delicious bokehs in the review of a rare lens Era-6M 50 / 1.5. And the background blur of the 35KP-1,8 / 65 is reminiscent of the bokeh of another, even more asymmetric lens of the “double Gauss” type - Zenitar 50/0.95 E.

Images of pre-focal spots (left side) and out-of-focal spots (that is, bokeh, right side) of the Era-1 50 / 1.8 lens with a maximum defocus of 3 mm. The angle is measured from the axis in the vertical direction.

35KP-1,8/65 has a very low level of vignetting on a 36×24 mm frame, it can be used without problems on 44×33 mm cameras or full-frame cameras with a tilt/shift adapter. On medium format cameras with a 44x33mm sensor, this 65/1.8 lens is exactly the equivalent of a 50/1.4 lens on small format. I posed for Andreeva Andrey with a Fujifilm GFX 50s camera to demonstrate the behavior of the lens in medium format.

I also took pictures on a full-frame mirrorless camera. Sony A7s using shift adapter

and without the use of a shift adapter.

All reviews of film projection and filming lenses:

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

Conclusions

The 35KP-1,8 / 65 film projection lens is another interesting lens of D.S. Volosov, as laconic, simple and unique both in terms of its parameters and the nature of the image as his other creations (time, two, three, four, five). The asymmetrized "Era" scheme he invented is a very delicate balance between ease of execution and image quality, which Volosov has always been able to find.

35KP-1,8/65 has convenient parameters, a beautiful uncomplicated pattern, easily adapts to modern cameras and can be especially useful for owners of medium format cameras. However, it must be remembered that the low culture of production at the IPP greatly spoiled this lens, therefore, in order to obtain a good result when shooting, each lens requires adjustment.

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