Review of the low magnification lens 2/0.05 160/- (no-name, China). Problems of constructing low magnification lenses for microscopes

Material on the lens especially for Radozhiva prepared Rodion Eshmakov.

Lenses for microscopes with magnifications less than 4x are rare. As a rule, such optics are not included in the standard package of microscopes and can be purchased separately. At the same time, there are a lot of macro lenses and camera accessories that allow you to achieve the same image scale, which gives rise to a number of questions: what is the difference between a 2x photographic lens and a 2x microscope lens, how and what non-specialized photographic lenses can be used to obtain a 2:1 image scale, and also why it is so difficult to make a high-quality 2x lens for a microscope.

This article presents an ultra-affordable ($10-$15) 2x lens for microscopes of the RMS standard with a tube length of 160 mm, the optical design of the microlens was analyzed and the image quality was compared with conventional photo optics. Some basic information regarding the use of microscopic optics on cameras is given here.

Technical specifications

Optical design – 4 lenses in 3 groups, Berthele telephoto lens;

Type of correction – achromat;
Tube distance – 160 mm;
Magnification factor – 2x;
Numerical aperture – 0.05;
Focal length – ~40 mm;
Relative aperture – ~F/8;
Working distance – 35 mm;
The thickness of the cover glass is 0 mm, the use of glass is optional;
Immersion required - no;
Mounting type – RMS standard (4/5” x 1/36” thread);
Features - microscopic lens, does not have an iris diaphragm and a focusing mechanism.

Design

The 2/0.05 lens body is made entirely of aluminum. The lens does not have any external removable parts - the design is monolithic. The lens has a bulk design without the use of an autocollimation assembly - the lenses are placed in the body without preliminary placement in special frames. Disassembly is carried out by unscrewing the rear splined ring. The front lens is deeply recessed into the body, which creates a kind of hood, but this recess does not have any blackening. Blackening is also present on the ends of the lenses; there is no matting on the interlens rings. In order for the lens to form a contrast image, these shortcomings must be eliminated when disassembling the lens.

Important: the middle lens of the objective has a biconvex shape, and its two surfaces are extremely similar to each other, and therefore the lens can very easily be assembled incorrectly. So easy that it may come from the store new, but already assembled incorrectly: take a look at the photo examples here – the focused image should be without a pronounced soft effect (“glow” of the contours), as well as at the appearance of the lens defocus spots – pre-focus and out-of-focus spots should be similar in appearance, without pronounced edges.

The optics of the 2/0.05 lens are coated with purple tints, most likely single-layer. The short-wavelength limit of light transmission is at ~365 nm. The coating does not introduce color distortion noticeable to the eye.

The optical design of the lens is similar to the Ludwig Berthele telephoto lens (US patent 2762262 1954). According to XRF analysis (Bruker M1 Mistral), both the front and rear lens elements are made of heavy crown glass. The negative lens in the gluing is niobium (lead-free) flint. The choice of these types of optical materials is generally consistent with the optical design shown in Bertele's patent. It cannot be said that this optical calculation has moved further towards more modern and interesting glass brands in terms of parameters.

 

The lens has very small dimensions and a long working distance of 35 mm. It is important that it is fully compatible with conventional microscopes with a parfocal distance of 45 mm optics, since the original Bertele lens does not fit into this limitation at all.

The following are photographs of the appearance of the 2/0.05 lens.

As you can see, the lens itself is very small in size, looks simple and budget-friendly.

Image quality

The image formed by the lens in the central area of ​​the frame is rather sharp, but there are still very noticeable spherochromatic aberrations. The lens field is close to flat, but the level of astigmatism is high, which is why even when refocusing, ideal sharpness at the edge of the field cannot be obtained. Compared to lens 4×0.1 Plan the picture with 2x0.05 seems softer.

The lens allows you to take photographs of objects that are quite large by microscopy standards. Some objects fit well into its entire field - a watch mechanism, for example, or small jewelry. The downside is that most transmitted light condensers of microscopes have a much smaller field, so the lens is convenient to use only in reflected light. Compared to 4x0.1 overview lenses, this one also has a noticeably greater depth of field, allowing you to shoot even without stacking. When using adapters like M42-RMS The lens can be mounted on a camera without a microscope for regular macro photography. Due to its fairly large working distance and small dimensions, this simple and cheap lens can be suitable for equipping microscopes for performing delicate work (important: “real” instrumental microscopes are stereoscopic, with an even larger working distance >80 mm) or for visual inspection equipment.

The following are examples of photographs taken with a 2×0.05 lens and a full-frame mirrorless camera Sony A7s, mounted on a modified NPZ M10 microscope at a tube distance of ~160-190 mm.

List of objects in the photo: 1 – elemental sulfur crystals, 2 – quartz watch mechanism, 3-5 – faceted synthetic single crystals of silicon carbide, 6 – faceted YAG:Nd:Ce crystal on two YAG:Nd, YAG:Nd:Ce garnet rods; 7-8 faceted YAG:Nd:Ce crystal in normal and UV illumination; 9-10 – faceted hydrothermal emeralds, 11 – faceted opal, 12 – micrometer object LOMO OMO-U4.2.

The following pictures were taken using stacking.

List of objects in the photo: 1 – crystals of elemental sulfur, 2-4 – faceted synthetic single crystals of silicon carbide, 5-7 – faceted hydrothermal emeralds, 8 – faceted YAG:Nd:Ce crystal on two rods of YAG:Nd, YAG:Nd garnets: Ce, 9 – faceted opal, 10 – crystals of potassium trisoxalatoferrate hydrate.

Macro 2:1 – microscope versus photo optics. Why is it so difficult to make a good 2x microscope lens?

As previously noted, 2:1 magnification is a typical working magnification for specialized macro lenses. Very often you can find decent examples of photographs at this scale, taken with ordinary lenses with macro rings or attachment lenses. To understand why there is a need to develop special lenses for macro photography and micro lenses, let’s consider how a regular photographic lens behaves when shooting at a 2:1 scale. To do this, we will simulate the optical design of a simple and understandable lens like Zenitar 50 / 2, the diagram of which is published in patent RU2290675C1 dated 2005.

The required image scale is achieved at a distance of 230 mm (from the object to the matrix). Let's set the numerical aperture to 0.05 (~F/8) and consider the graphs of longitudinal aberration, field curvature, frequency-contrast characteristics and the diagram of aberration spots.

Already from the spot diagram you can see how quickly the optical quality of the lens decreases with distance from the optical axis due to the influence of field curvature and astigmatism. In this regard, he turns out to be no better the simplest doublet lens. In the center, a resolution of 50 lines/mm is achieved.

At F/8, this lens has no problems with image quality when focusing at infinity. The observed deterioration is due to the fact that, in essence, the lens does not work correctly: it is designed so that the image being formed is close to the rear lens, and the object is as far away from the front lens as possible. In this case, the opposite is true: the object is closer to the front lens than the image being formed is to the back one. Therefore, in order to slightly change the situation for the better, the lens needs to be turned backwards. For this there are special reversible adapters, and now you can clearly see why they should be used.

Below are the simulation results for the same lens under the same conditions, but upside down.

It is easy to see that the field curvature and astigmatism in this case are much lower, due to which the image quality across the field has increased significantly, although it is still far from ideal: the lens is also not designed for shooting at a 1:2 scale.

To solve this problem, it is enough to choose a reproduction lens as a reversal - for example, Vegu-11U. This lens provides the best image quality at a scale of ~1:4-1:2, inverted, therefore 2:1-4:1.

Indeed: Vega-11U is capable of providing good quality both in the central image area and at the edge of the field with an inverted aperture of ~F/8. It just turns out that such a lens is not at all suitable for use in a microscope due to the non-compliance of the parfocal and tube distances with the standards. The parfocal distance of such a lens is twice as large as required (standard 45 mm), and the tube distance is 20% less than required (the lens will need to be “recessed” deep into the microscope). To increase the tube distance and reduce the parfocal distance, the use of an inverted telephoto lens design is required - a retrofocal optical design.

You can try the inverted Bertele telephoto lens (US patent 2762262 1954) as such a lens. It is, of course, not designed for use on such a scale, but simulation will allow us to see how it meets the requirements for microscopic optics.

With a tube distance of 160 mm, this lens has a parfocal distance of 61 mm, which is already much better than 90 and suitable for some microscopic systems (Nikon, for example). But the lens is still not suitable for most conventional microscopes. In addition, you can see how much worse the quality of this telephoto lens is in comparison with the Vega, which is of similar complexity: teleshortening the system is always a compromise in image quality.

I was wondering how difficult it would be to design a 2x microscope objective with a parfocal distance of 45 mm that could provide good quality across a 36x24 mm frame and a resolution of ~50 l/mm. The design of the Berthele telephoto lens was taken as a basis; modern optical materials from the CDGM catalog were used in the calculations.

The result of the calculation was a six-lens apochromat plan (400-700 nm), which fully meets the requirements of the RMS standard with a parfocal distance of 45 mm and a tube distance of 160 mm. The lens has a long working distance and a compact optical design.

As it turned out, when focusing such a lens at infinity, the working distance is 220% of the focal length - approximately the same ratio is achieved in complex wide-angle lenses such as 20/3.5 for SLR cameras. Calculating microscopic lenses of low magnification is a task of comparable complexity. That is why, in order to improve the quality of the Bertele lens while reducing the dimensions, it was necessary to increase the number of lenses and use the most modern optical materials, primarily super-heavy flints with a refractive index of up to 1.95.

The Chinese 2×0.05 lens discussed in this article does not use a complex optical design and/or advanced materials, and therefore has a very moderate on-axis resolution and low image quality over a field that is approximately half that of a 36×24 frame.

 

All reviews of RMS standard microscope lenses with a tube distance of 160 mm:

Modern optics from Chinese manufacturers:

1. Review of the low magnification lens 2/0.05 160/- (no-name, China). Problems of constructing low magnification lenses for microscopes
2. 4x0.1 160/0.17 achromat (China, no-name)
3. Microscopic optics on a camera. Review of microscope lens Plan 4x0.1 160/0.17 (China, no-name)
4. 10x0.25 160/0.17 achromat (China, no-name) - modification and test
5. Review and comparative test of microscope achromat 20/0.40 160/0.17 (China, no-name)
6. Review of the Planachromat microscope lens Plan 20x0.4 160/0.17 (no-name, China)

Reviews of Soviet lenses for microscopes:

1. Microscope objectives 3.7x0.11 (OM-12), 4.7x0.11 (LOMO, Progress): review and test
2. Review and test of the LOMO M42 8x0.2 achromatic microscope
3. Review, analysis and large comparative test of microscope objectives LOMO Plan 9x0.20 and 10x0.20 (OM-2)
4. LOMO Epi 9x0.2 (OE-9, adapted)
5. LOMO 10x0.4 L (OM-33L) - modification and test
6. Review and test of the OM-27 20x0.4 (Progress) achromatic microscope
7. Review of achromat microscope lens LOMO 21×0.4 190-P (OM-8P)

Conclusions

Simple microscope lens 2/0.05 160/- is a working and practically the only available solution for those who would like to get a large working field on a conventional biological microscope. The lens is also suitable for macro photography, but if compatibility with a microscope is not required, then it is better to purchase reversible adapter with a set macro ring or specialized macro lens.