Review of the lens for technical cameras Witrue HD 75mm 1:2.8 C-mt 10 Mega Pixels

Material on the lens especially for Radozhiva prepared Rodion Eshmakov.

Witrue HD 75/2.8 on Sony A7s.

A lens without the name "10Mega Pixels 75mm 1:2.8 C-mt", sold as Witrue HD 75/2.8 or Chuan Wei 75/2.8 – is a modern, simple and affordable (~$40) technical lens for machine vision equipment and other special cameras with a 1” matrix format and C-mount. Reviews of similar lenses have already been published were on the site.

Some technical lenses with C-mount have proven to be easy to install on mirrorless cameras via adapter, very low cost and interesting optical properties were so popular that they were released as photo lensesThe lens presented in this article, which I will call "Witrue HD 75/2.8", also has interesting characteristics and its popularity is probably yet to come.

Specifications:

Optical design – probably 4 elements in 4 groups, Ernostar/Zonnar type, at least 1 element made of high refractive index glass (n>1.8);

Presumed optical design of the lens. No official information.

Focal length - 75 mm;
Relative aperture - 1: 2.8;
Frame format – 1” (13.2×8.8 mm, calculated), APS-C (23.6×15.8 mm, covered);
Field of view angle – 11° (calculated for 1” matrix), 20° (for APS-C frame);
Aperture - 6 blades, without a preset mechanism;
Aperture limits - 1:2.8 - 1:22;
The minimum focusing distance is 1.2 m;
Thread for light filters – М35.5×0.5;
Camera mount – C-mount;
Overall dimensions – 39×46 mm (excluding camera mounting thread);
Weight - 120 g;
Total length of the optical system – ~61 mm (telephoto shortening ~19%);
Features include screws to lock the position of the focusing rings and aperture setting.

Lens specifications from the seller's page.

Lens design

Despite its low cost, the Witrue HD 75/2.8 is made entirely of metal and glass. The lens has a fairly high-quality assembly, some parts are even fixed with varnish at the factory - for example, the nut at the front lens. The markings on the lens are very poor: there is no brand, no country of manufacture, there is not even a focusing distance scale - only "Closer" / "Farther". A modest technical lens - what else can I say.

The most important feature of the Witrue HD 75/2.8 is its dimensions: so far, it is the smallest 75 mm lens I have ever come across. And this concerns not only the dimensions of the lens block, but also the overall length of the system, which determines the dimensions with the camera: it is almost 20% shorter than the focal length! This is very significant, because, as a rule, such lenses have a length equal to or greater than the focal length (Sonnar F/2.8 - 96% of the focal length, Triplet F/2.8 - 115% of the focal length, Tair F/2.8 - 103% of the focal length). Such shortening is usually achieved by using a telephoto lens layout and is generally a compromise with optical quality and system complexity, but in this lens the result is achieved using a scheme similar to the Sonnar/Jupiter lenses, which are not telephoto lenses by design.

Almost the entire side surface of the lens body is occupied by two rings of the same thickness for controlling the aperture and focusing. The rings do not differ in any way by touch and are very easy to confuse, which causes inconvenience when using them on a camera. Of course, with the intended typical use of the lens in special products, this is unlikely to bother anyone: apparently, the positions of the focuser and aperture are often fixed with the included screws, holes for which are provided in both rings.

When focusing, the lens extends by ~6.5 mm, the lens block does not rotate. The angular stroke of the focusing ring is quite small - only 90°. Even with an excessive minimum focusing distance of 1.2 m for such a focal length, the focusing ring seems too sensitive to the slightest movement. To somehow reduce the MDF, I often used macro rings with C-mount threads, which is not very convenient due to the need to constantly screw and unscrew the threads.

The lens diaphragm has only 6 blades, forming a hole of a regular hexagonal shape. The blades have a matte black finish. The diaphragm is controlled steplessly. The lens body is marked with aperture values ​​in whole steps from F/2.8 to F/22, with a skipped F/11 value and a dotted F/4 designation. The aperture number markers are far from ideally consistent with the actual value, if you compare pictures with different apertures according to exposure.

According to transmission spectroscopy data, the objective lenses are coated with a two-layer anti-reflective coating. The color of the highlight is green, pink, blue-green. The spectrum profile has an uneven shape and clearly consists of two bands with peak transmission values ​​of 470 and 650 nm, which corresponds to the typical assignment of the visible spectrum for visual devices. In the green region at ~530 nm, there is a clear transmission dip. This is unimportant for a technical lens, but is a serious drawback for a photographic lens: an uneven transmission spectrum with a complex profile leads to difficult-to-eliminate color distortions ("dirty color"). Lenses with high-quality coatings have no such disadvantages. Sometimes the coatings with large color distortions in general, the final color rendering is less affected due to simple compensation by means of the task white balance cameras.

Light transmission spectrum Witrue HD 75/2.8.

There is no official information about the optical design of the lens, or the materials used in it. However, upon partial disassembly of the lens, it was possible to find out that, apparently, the lens is made according to a four-lens design of the "Sonnar"/"Ernostar" type. It is not entirely clear whether these are four separate lenses or the design is similar to the lens Jupiter-37A, eg.

An X-ray fluorescence spectroscopy study of the elemental composition of the front lens of the objective lens (Bruker M1 Mistral) showed that it is made of heavy crown glass type TK14, most likely H-ZK9B 620.603, as indicated by the detectable barium, zinc and strontium.

XRF spectrum of the front element of the Witrue HD 75/2.8 lens.

More interesting is the material of the rear lens: large amounts of niobium point to a glass of the heavy flint type H-ZF52 847.238 or even H-ZF62 923.209, which have an ultra-high refractive index, very high dispersion and a very high positive deviation of the partial dispersion in the blue region of the spectrum from the normal straight line (similar to special crowns - the so-called ED glasses). In simple optical schemes, the use of niobium high-dispersion high-refractive flints in positive lenses is often a good way to correct spherochromatic aberration.

XRF spectrum of the rear element of the Witrue HD 75/2.8 lens.

In a sense, heavy and super-heavy lead-free flints play the same role as anomalous low-dispersion crowns. Lead glasses are also suitable, although to a lesser extent - and the last lens of a similar objective design Jupiter 6 was made from TF7 728.283 glass, which belongs to the class of heavy lead flints.

Thus, the Witrue HD 75/2.8 lens, although simple, has a good anti-reflective coating and fairly modern optical materials. But there is one thing this lens has been seriously shortchanged - the quality of the blackening. The sleeve between the last lens and the lens diaphragm does not have high-quality light protection and is very glare-producing, which greatly affects the overall contrast of the image - and it is unlikely that a lens hood can help here. I largely eliminated this drawback by blackening both the sleeve and the space behind the lens with matte black paint, removing the rear lens. Otherwise, the lens rather pleasantly surprised me, especially, of course, with its compactness.

Optical properties

The image formed by the lens in the frame area corresponding to the 1”-4/3” format is sharp with the aperture open. The lens has a good degree of correction spherical aberration and longitudinal chromatic distortions, has no focus shift, does not suffer from noticeable coma or astigmatism. Within the calculated field, distortion is also well corrected, which is essential for use as a technical lens. Outside the calculated field on an APS-C frame, the quality decreases due to astigmatism and noticeable lateral chromatic aberrations of green-purple color. Stopping down to F/5.6-F/8 allows to improve the image quality to some extent, but lateral chromatic aberration cannot be overcome this way. Below are examples of photos taken at apertures of F/2.8-F/8 on a Sony NEX-6 (APS-C) camera, as well as 100% crops of photos.

The image contrast before blackening the shiny sleeve with the last lens is mediocre, and in backlight is unacceptable. Blackening allows to significantly improve the result and returns the lens to its quality: the lens already works very well in normal lighting and quite well in backlight. However, Witrue HD 75/2.8 at certain angles catches very strong glare from the sun, which fills the entire frame with light. This is probably not related to blackening, but is a property of the optical scheme itself.

The lens's color rendering seems quite decent, although in my opinion it does something wrong with green shades due to its transmission spectrum. In any case, working with this lens is actually still easier than with the ancient Soviet ones with their bright spin-coating coating, which introduces a dense color veil.
The lens's bokeh wide open is commendable. It is virtually uniform across the frame, with the exception of some clipping due to vignetting. Some low-level residual higher-order aberrations give the blur its characteristic Sonnar type lenses view.

Below are some sample photos taken with Sony cameras. NEX-6, NEX-3Women, A7s (in APS-C mode). C-mount extension tubes were used for some photos.

Conclusions

I really liked the tiny Witrue HD 75/2.8 in combination with the compact Sony NEX-6. The lens has sufficient aperture, high sharpness across the entire frame except perhaps the corners, excellent background blur. And all this for a very small price for a new lens.

10 main advantages:

1. Ultra-small dimensions – almost any other 75mm lens will be longer and heavier;
2. Modern optical materials the lens design uses highly refractive glass;
3. Good image sharpness over most of the frame;
4. Low distortion level;
5. Beautiful background blur, reminiscent of Sonnar lenses;
6. High achievable image contrast due to the quality of lens coatings;
7. Universal C-mount, easy to install on any mirrorless camera;
8. Better color rendering compared to many older lenses;
9. The focusing method does not prevent the use of polarizing and gradient filters; the focus or aperture position can be fixed with screws;
10. Low cost (~ 40 $).

10 main disadvantages:

1. Poor quality blackening of the sleeve in front of the last lens of the objective requires correction to achieve high image contrast;
2. The lens captures frame-filling flares in backlit situations quite easily;
3. High level of lateral chromatic aberration and astigmatism outside the frame field ~4/3”;
4. The minimum focusing distance of 1.2 m is excessively large for such a focal length (the typical value is 0.8 m);
5. Only six aperture blades – “nuts” appear in the bokeh;
6. Very similar focus and aperture control rings - easy to confuse, takes some getting used to;
7. Small angular travel of the focusing ring – too sensitive, easily knocked off focus;
8. Quite high vignetting on the APS-C frame at open aperture;
9. Compromise lens coating that does not meet the high standards of coatings for modern photographic equipment;
10. Modest by today's standards aperture for 75mm lens.