Review of vintage rangefinder lens FED 1:3,5 F=50 mm

Material for FED lens 1:3,5 F=50 mm especially for Radozhiva prepared Rodion Eshmakov.

FED 1:3,5 F=50 mm lenses (hereinafter referred to as FED 50/3.5) were equipped with FED and FED-2 cameras - Soviet unlicensed copies of German Leica rangefinder cameras, which were produced in Kharkov at the FED machine-building plant, named after the security officer Dzerzhinsky. The FED plant has a unique history associated with the name of the famous teacher Anton Semenovich Makarenko, who, on the basis of a small commune for street children, organized the production of first hand-held drilling tools, and then film cameras (the first FED cameras). Subsequently, production expanded, and the labor commune was reorganized in 1939 into the Dzerzhinsky plant belonging to the NKVD (cameras of the FED NKVD series). The plant produced not only cameras, but also a whole line of lenses, most of which in design were interpretations of the optical designs of German designers Willy Merthe, Robert Richter and Paul Rudolf:

• Standard lens FED 50/3.5 (presented in this article);
• Wide-angle symmetrical six-element lens FED 28/4.5;
• Stereo lens FED 38/3.8;
• Macro lens FED-19 59/3.5;
• High-aperture FED 50/2 ;
• Telephoto lenses FED 100/6.3 and 100/5.9.

In a sense, the line of optics for pre-war and wartime FED cameras was even richer than for Soviet post-war rangefinders: it contains highly specialized lenses that were not developed in the post-war years.

The most common of the lenses listed is the standard FED 50/3.5, which also has the name “Industar-10” and was produced for a long time (at least from the 1930s to the 1950s) by many different enterprises (FED, KOMZ, VOOMP, GOMZ , “Geodesy”) in different versions (photographic, technical). Industar-10 is known as the standard lens of the world’s first small-format SLR camera “Sport” / “Helvetta” (prototypes - 1934). And in general, this is the first Soviet small-format lens of the Tessar type. Industar-10 lenses produced before the late 40s and early 50s do not have an anti-reflective coating on the optics, so we can assume that the coated FED 50/3.5 presented in this article was released in the period after ~1948.

Specifications:

Optical design - 4 lenses in 3 groups, "Tessar";

Focal length - 50 mm;
Relative aperture - 1: 3.5;
Aperture - 8 blades;
Aperture limits - 1: 3.5–1: 16;
The minimum focusing distance is 1 m;
There is no thread for filters;
Mounting to the camera – M39 thread (working distance 28.8 mm);
Features – folding lens, lens unit is parked deep into the camera to reduce dimensions; Infinity focus position lock.

Design and execution of the lens

FED-50/3.5 has an interesting and unusual design by today's standards. Firstly, the lens itself is not made of aluminum, as usual, but of brass, coated with nickel (front surfaces) and chrome (tube). In my opinion, nickel plating looks great - beautiful, but discreet.

Secondly, the lens copies the folding design of early Leitz Elmar: the FED 50/3.5 lens unit is mounted in a movable tube that can move inside the focusing mechanism. In this case, in the working position, the tube extends and is fixed by turning it clockwise, and in the transport position, the tube is completely recessed into the focuser and camera shaft, making the lens very compact.

Subsequently, this design was used in later Industar lenses for rangefinder cameras. Reviews of other Industar folding lenses:

• Industar-50 3,5 / 50 (LZOS, 1969);
• INDUSTAR-22 1:3.5 F=50mm. Moscow (KMZ);
• Industar-22 1: 3,5 F = 50mm P (KMZ) here и here.

According to my observations, the FED 50/3.5 does not catch on the inside of the camera (Sony A7s) when folded, unlike Industar-50. When folded, the camera-lens combination is very compact and can even fit into a jacket pocket.

The disadvantage of the design is the inconvenient aperture control, which is carried out by a thin ring at the front lens of the lens, equipped with a small finger rest. Very often this ring corrodes, becomes clogged and does not rotate well - such a lens needs to be disassembled and cleaned.

The iris diaphragm mechanism consists of 8 dark matte rounded blades. At closed apertures, the lens pupil takes the shape of a regular octagon with rounded corners.

The focusing mechanism of the lens is an open type, that is, the multi-start helicoid thread is exposed when focusing at close distances. This focuser is as easy to maintain as it is to get dirty. The minimum focusing distance (MDF) is 1 meter, which, of course, is very long for a 50 mm lens. However, by replacing the focuser stop screws located on the distance scale, you can reduce the MDF to 0.5 m. In this case, the helicoid will perform one full revolution, and not half.

An interesting feature of the lens is the presence of an infinity focus position lock, made in the form of a button. This element also serves as a support for the finger when focusing in the only possible way - there is no special ring for this in the design of the FED 50/3.5.

There is no great benefit from the lock, although the option will probably be useful when shooting infinitely distant objects at small apertures. On the other hand, I would prefer not to trust the lock and still refocus - the lens is characterized by focus shift (changing the focus position with aperture).

Finally, the most important nuance of the lock is the problem that arises with focusing when installing the lens on modern cameras via adapters. The lock button requires a downward movement of ~2 mm beyond the plane where the lens is attached to the camera, and therefore the button usually rests against the adapter and it is impossible to remove the lock. To use the lens, I had to drill out the adapter in the right place to create enough space for the clamp to work. However, this design element has been preserved in working condition in a very small number of Industar folding lenses - often this button is no longer in place.

FED 50/3.5 is not Ultron 28/2, and scolding it for its inconvenient mechanics would not be entirely correct, which does not negate the fact that the lens can be problematic when used with modern cameras. At the same time, FED 50/3.5 catches the eye with its vintage finish and unusual design.

Optical design. Development of lenses like Tessar 50/3.5

The FED-10 50/3.5 lens is made according to the Tessar optical design, invented by the German optician Paul Rudolf. The development of this scheme was carried out by Ernst Vandersleb and Willy Merthe. Back in 1926, in Germany, Willy Merthe applied for a patent for a Tessar-type lens with a relative aperture of 1:3.5 (US 1741947). The Soviet Industar-10 appeared almost 10 years later, Industar-22 - in the late 40s, and Industar-50 - in the 50s. Of course, it is of interest to compare the optical design of Soviet lenses with the original German ones and with each other.

The comparison was carried out using modeling in the ANSYS Zemax 13 software package for Industar-22m lenses (see about I-22 and I-22m here), Industar-50 and Tessar 50/3.5 (US 1741947). Unfortunately, it was not possible to find documentation for modeling the optical design of Industar-10, although it can be considered optically close to Industar-22.

The image below shows a schematic diagram of the lenses in question, as well as the brands of optical glasses used and their parameters (refractive index and Abbe number, which characterizes the dispersion of the material).

Note that the original Tessar 50/3.5 uses glass with a slightly higher refractive index for positive lenses than the Soviet versions. In addition, it turned out that Industar-22m and Industar-50 differ only in the brands of glass in the positive lenses (TK16 in I-22m and TK14 in I-50), but at the same time they have exactly the same geometric parameters of the lenses and their position.

To determine the type of glass in the front and rear lenses of the FED 50/3.5 (Industar-10) lens, I recorded X-ray fluorescence spectra (XRF, XRF, XRF) using a Bruker M1 Mistral device, which turned out to be generally identical.

Tin (Sn) and zirconium (Zr) are elements that are constantly present in the spectrum for this device, and therefore were identified erroneously. The copper (Cu) signal is a consequence of the interaction of radiation with the lens frame. The data indicate the presence of barium (Ba), zinc (Zn), strontium (Sr) and antimony (Sb) in the glass composition, which corresponds to TK type glass (heavy crown) and is similar to the spectrum of TK16 glass. Consequently, the FED 50/3.5 lens (Industar-10) is very close to the Industar-22m lens.

To compare lenses with each other, the following were calculated:

• Diagrams of aberration spots when focusing at infinity on an open aperture - for visual assessment of the performance of the circuit at a qualitative level. The larger the spot, the worse it is, as a rule. The actual quality also depends on the structure of the spot itself and the distribution of energy in it.
• Lens longitudinal aberration graph for wavelengths from 400 to 700 nm - demonstrates the influence of spherical and chromatic aberrations for a point on the optical axis at different apertures. Along the vertical axis is the size of the lens pupil (aperture), along the horizontal axis is the focus position. For an ideal lens, the bundle of curves is infinitely narrow and coincides with the vertical axis.
• Graph of the distortion value in % depending on the image height coordinate.
• A graph of field curvature and astigmatism is necessary to assess the quality of the lens across the field, especially at small apertures. In an ideal lens, the beam of curves is infinitely narrow and coincides with the vertical axis. Astigmatism is a divergence of the beam by two, field curvature is an inclination relative to the vertical axis.
• Frequency-contrast characteristic curves (MTF, MTF) for frequencies 10 mm-1 (contour sharpness) and 30 mm-1 (sharpness in details) across the image field at F/3.5.

The results are presented in the pictures below.

So, the original Willy Merthe lens, as it turned out, has good correction of chromatic aberrations, has a low level of distortion (0.5%, “barrel”), but astigmatism and field curvature are not sufficiently corrected, as a result of which the image quality across the field drops quite significantly at an open aperture. On the other hand, the sagittal and tangential focuses of this lens do not intersect (see field curvature diagram), and therefore the lens has a low level of astigmatism in the corners of the 36×24 frame, which cannot be said about the Industar-50 and Industar-22 lenses.

Industar-22 and Industar-50, differing in design only in that in Industar-50 TK16 glass was replaced by TK14 glass with the same refractive index, but less dispersion, and in the picture they differ only in terms of correction of chromatic aberrations. In the Industar-22 lens, chromatism is poorly corrected, the secondary spectrum is one and a half times higher than in the original Tessar 50/3.5 Merte, but Industar-50 has a rather low level of chromatism. In general, in Industar-50 chromatism is corrected even better than in the popular Industar-61.

It is clearly seen from the MTF graphs that the transmission of fine details (contrast at a frequency of 30 mm-1) strongly depends on the degree of correction of chromatic aberrations. So, the leader is Industar-50, followed by Tessar 50/3.5, the outsider is Industar-22. In addition, the maximum achievable resolution at apertures F/5.6-F/11 also depends on correcting chromatism.

As for the image quality across the field, despite the fact that all lenses are very close to each other in terms of MTF at the edges of the frame on an open aperture, the situation will be different on apertures F/8-F/11: diagrams of field curvature and astigmatism show that for Due to the very high level of astigmatism, the Industar-22 and Industar-50 lenses make it impossible to achieve high image quality in the corners of the frame, while the original Tessar 50/3.5 allows this. According to the results of live testing, the FED 50/3.5 lens behaves in a manner similar to the Industar-22 lens.

Thus, the Soviet Industar-50 lens - the last large-scale Tessar 50/3.5 - is ahead of the original lens in sharpness in the central region of the frame at all apertures, but the corners of the frame have been sacrificed. Industar-22 and, apparently, FED 50/3.5 (Industar-10) are even “softer” lenses in comparison with the German Tessar 50/3.5, with pronounced chromatic aberrations and the same problematic frame angles as the Industar-50.

Optical properties

The FED 50/3.5 really showed itself to be a rather soft, not very sharp lens. The image has a low level of detail even in the central area of ​​the frame, although the contour contrast is quite acceptable. The same problem was typical for the lens Industar-29 80 / 2.8, which is essentially proportionally increased Industar-26m, which itself has a very large secondary spectrum.

Across the frame field, the FED 50/3.5 has low image quality down to F/5.6-F/8, and the frame angles are never ideal due to astigmatism.

The image contrast under normal conditions is at the level of other similar lenses with single-layer coating of optics. In backlight, veiling appears; the lens catches rainbow highlights and other interesting artifacts. Due to the lens coating used, the lens transmits the blue part of the spectrum somewhat worse (i.e., slightly yellow), but has high light transmission in the near-IR region. In general, it is difficult to notice any specific effect of the coating on color rendition.

FED 50/3.5 has a pleasant background blur when shooting close-up objects and portraits. The bokeh of the lens is twisted due to the influence of astigmatism and geometric vignetting, but it does not look as tacky as that of Helios-44, for example.
Below are examples of photographs taken with a full-frame mirrorless camera Sony A7s https://radojuva.com/2022/06/a7s/#more-79695.

Conclusions

FED 50/3.5 is the first Soviet small-format anastigmat. It's hard to ask much from this lens. The FED 50/3.5 has poor optical quality and not the most attractive parameters, but in itself it is a rather interesting lens in terms of both tactile sensations when used and the results obtained when shooting. Considering the low price of the lens on the secondary market, the FED 50/3.5 can be confidently recommended to hunters of real vintage.

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