RADOJUVA

Material on the Zenit IR-Tele 2,8 / 135 lens especially for Radozhiva prepared Rodion Eshmakov.

The lens was provided by Alexander Korolev especially for the review (Inst: revvoensovet).

This article presents a lens that has never been designed for cameras - a lens from a night vision device (NVD) Zenit IR-Tele 2,8 / 135 manufactured by KMZ. Because NVD image intensifier tubes are sensitive to the visible orange to near-infrared spectrum, the optics for use with them are generally not designed to operate in the short-wavelength visible spectrum and therefore produce a rather unusual image on conventional cameras.

Other reviews of night vision lenses adapted for use with conventional cameras:

• 50mm f/1.5 lens of the tank binocular TVNO-2B (IPZ, Izyum, Ukraine)
• 50mm f/1.2 lens of the tank binocular TVNE-4B (IPZ, Izyum, Ukraine)
• 37mm f/1.0 lens of night vision goggles PNV-57E (LZOS, Lytkarino, RF)
• 130mm f/1.8 mirror-lens lens PNV Cathod-MNV (refinery, Novosibirsk, RF)

Specifications (according to the KMZ documentation and patent RU2047200C1):

Optical design - 5 lenses in 4 groups;

Focal length - 135 mm;
Relative aperture - 1: 2.8;
Spectral range - 680–880 nm;
Diaphragm design - 8 blades, stepless;
Linear field of view (calculated) - 18 mm (covers APS-H without vignette, and full frame - when focusing on MDF);
Mount - M42 (rear focal length ~ 36 mm does not allow to achieve infinity focusing on full-frame SLR cameras);
The minimum focusing distance is 5 m;
Dimensions - 57 x 91.5 mm;
Weight - 0.5 kg;
Features - internal focusing.

Design features

Zenit IR-Tele 2,8 / 135 (hereinafter referred to as simply Zenit IR) is made in a body that is functionally similar to conventional photographic lenses: there are quite familiar focus control rings and aperture settings, moreover, rubberized. The only caveat is that there are no familiar marks on the aperture scale on the aperture scale, but only an indication of the “brighter-dimmer” type.

The iris diaphragm of the lens itself consists of 8 petals and forms a “circular saw” pupil when closing.

As you can see, the lens turns very yellow through the light - this is due to a special single-layer anti-reflection coating that provides maximum light transmission in the red region of the spectrum, but reflects blue light - very similar to the coating used in TVNO-2B 50/1.5.

A feature of the lens is internal focusing. The lens mounted on the camera does not change its dimensions when focusing, only the rear lens moves, and not the entire lens block. However, this method of focusing in simple optical designs can greatly affect image quality, especially at the edges of the frame. It is for this reason that the Zenit IR has a very large minimum focusing distance of as much as 5 meters.

By itself, the rear lens of the lens, when focusing to infinity, strongly protrudes beyond the M42 mount (exactly the same as for Zenith cameras). It can hook the mirror on full-frame SLR cameras. Moreover, the size of the rear lens of the lens is too small to cover the 36x24mm matrix well.

There are no particular complaints about the build quality and performance of the lens, although it is clear that the focusing method used is connected only with the desire to simplify and reduce the cost of the design. It is also worth noting the problem, traditional for Soviet / post-Soviet optics, with the quality of the blackening of the spaces between the lenses.

Optical properties

I had the opportunity to test the Zenit-IR lens only in the visible range (400-700 nm), since there was no full-spectrum camera in stock. In general, the image quality on a conventional camera can be assessed as unsatisfactory at any aperture set due to the strongest longitudinal chromatic aberrations - the lens is completely not designed to work in the visible spectrum. The best results can be achieved when shooting subjects that do not have a blue part of the spectrum, which is, oddly enough, portrait photos taken with this lens look satisfactory.

On a full frame camera, the lens vignettes heavily when focusing at infinity. Aperture only exacerbates the effect. To get around this problem, you can use a lens on a mirrorless camera with an adapter equipped with its own helicoid - in this case, you can mount the lens on a 5m MDF and focus it with a second helicoid to infinity, thereby avoiding strong vignette. Below are examples of photos on a covered aperture (about F / 5.6), illustrating this effect on the Sony A7s.

At closer ranges, the effect is less pronounced, with vignetting contributing significantly to the shape of the lens' bokeh.

The color reproduction of a lens is highly distorted by both light transmission and light scattering on glass-to-air surfaces. The resulting image has a yellow tint, but in the backlight, a dense blue veil often also appears, which creates an unusual combination of complementary colors in the frame. At the same time, the image contrast is low, in the backlight it is low.

In the visible region of the spectrum, the lens has an expressive bokeh. Out-of-focus discs have bright orange fringing due to the effect of chromatic aberration. Because of the vignetting along the edge of the frame, the discs turn into characteristic "lemons".

Since the optical design of the lens is known and published in patent RU2047200C1, I simulated its optical properties in ANSYS Zemax 13 software to clarify the behavior of the lens in the visible region (400-700 nm, RGB spectral function of CMOS camera matrices) and in the range of 600-900 nm (orange - near IR).

As you can see, in the calculated spectral range, the lens forms an image of quite acceptable quality both on the axis and in the field. However, in the visible region, the sizes of aberration spots increase by a factor of ~4–6, as well as chromatic aberration.

The following are sample photos taken with the Sony A7s full frame camera.

It seemed to me that many of the photos taken with this lens look interesting in black and white. Below are some of them.

Update

Shared unusual photos with Zenit IR 135/2.8 and Sony NEX-3 converted into a full-spectrum camera @concrete walls Sergey Shcherbakov.

Was used for shooting Zomei IR filter 720 nm (does not transmit light with wavelengths shorter than 720 nm - i.e. most of the visible spectrum). The color gamut (“false color”) of photographs is determined by the different sensitivity of the matrix subpixels to long-wave radiation. Under the indicated shooting conditions, the lens is superior in image quality to the Jupiter-37A, including at an open aperture.

Conclusions

Zenit IR-Tele 2,8/135 is a highly specialized lens not intended for conventional cameras. In the visible spectrum, it forms an interesting, but extremely low-quality image, while Zenit IR-Tele 2,8 / 135 fits very well for shooting in the IR range.

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