Modification and test of achromat microscope lens 10x0.25 160/0.17 (China, no-name)

Material especially for Radozhiva prepared Rodion Eshmakov.

This article presents the simplest and cheapest ($10) from the microscope lens without a name with a magnification of 10x found on the vastness of the Chinese marketplace. Modern entry-level microscopes with a tube distance of 160 mm and an RMS mount are equipped with such lenses in very different external versions, and therefore, probably, for many beginning microscopists this lens will be the first tenfold lens. As it turns out, even an ultra-budget solution can serve well, especially after modifications and modifications, which will be discussed below. Some information about microscopic optics, terminology, classification, methods of application is given here.

Technical specifications

Optical design – 4 lenses in 2 groups (type aplanata Richter), without the use of special elements;

Type of correction – achromat;
Tube distance – 160 mm;
Magnification factor – 10x;
Numerical aperture – 0.25;
Lateral chromatism (increase chromatism) – ~0%;
Focal length - 16 mm;
Relative aperture – ~F/2;
The estimated image field size is 18 mm;
Parfocal distance – 45 mm;
Working distance – 7 mm;
Cover glass – 0.17 mm (in fact, its use 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.

Lens design and modification

The 10x0.25 lens has a metal body and a black color scheme. Unlike more expensive lenses from the line Plan 160/0.17 Some parts are made of plastic: the lens aperture diaphragm (clear diameter - 9 mm) and the ergonomic ribbed ring. Markings indicating lens parameters are applied to the black removable outer ring (“shirt”) with paint, without engraving.

The dimensions of the lens are similar to other 10x lenses, in particular LOMO 10×0.4 L, but the design of the lens block is similar to the Soviet 8×0.2 achromats: the lens does not have any complex lens centering system.

For disassembly, simply unscrew the plastic diaphragm and the rear slotted nut - after this, you can remove the two lens components in the frames and the metal interlens insert. The optical design of the lens is similar to the Soviet one 9×0.2 Epi, but uses more modern optical materials. So, according to X-ray fluorescence analysis (Bruker M1 Mistral), the negative lenses of the lens are made of niobium flints from the “environmentally friendly” line of glasses such as CDGM HF or H-ZF, and the positive lenses are made of heavy crowns of the CDGM H-ZK type.

 

 

In the case of the Richter scheme, the use of “lead-free” HF/H-ZF glasses in negative lenses is extremely undesirable, since the value of the relative partial dispersion PgF for such materials is much, much higher in comparison with similar “outdated” F/ZF lead glasses. Since the magnitude of longitudinal chromatic aberration can be estimated as the difference between the PgF values ​​of crown and flint, related to the difference in Abbe numbers, it is clear that a lens using lead flints for the manufacture of negative lenses will have less chromaticity than the same lens on lead-free glass. This effect is so important that the widespread transition to the use of lead-free glass in the 1990s led to the need to recalculate many old optical circuits, which no longer provided an acceptable level of quality after replacing glass brands.

To compensate for spherochromatic aberrations that are inherent in lenses with a aplanata Richter, an aperture diaphragm with a light diameter of 3 mm was manufactured on a 6D printer, which approximately halves the relative aperture of the lens. Of course, this leads to an increase in diffraction effects, but, as will be shown later, it is spherochromatic aberrations that limit the resolution of this lens, and not diffraction.

 

It turned out that there is no matte blackening on the internal surfaces of the 10×0.25 lens, and the frame of the front lens is completely made of shiny metal without blackening the ends of the lens. I didn’t even tempt fate and first of all I covered all the glossy surfaces with matte black paint: the lens frames, the interlens insert. If you skip this step, then, for sure, the contrast of the image will be extremely weak.

It’s funny, but they didn’t forget about the anti-coating of the optics in this lens: all the surfaces of the lenses of this Chinese 10x0.25 are purple. The presence of antireflection coating on the optics is a big plus, even if the blackening was successfully “optimized” during production: you can blacken this lens yourself, but applying antireflection coating at home simply cannot be done.

 

The coating, designed for use in visual devices, provides a transmission peak in the green region of the spectrum and slightly cuts off the blue-violet and infrared spectral ranges. However, this is only noticeable to laboratory instruments. The shape of the curve suggests that the clearing consists of 1-2 layers. The short-wavelength transmission limit is approximately 350 nm.

From the point of view of ease of use of the lens, it is also important that this lens has a fairly large working distance of 7 mm. This is quite enough to supply side lighting for working in reflected light.

Photos of the lens's appearance are shown below.

 

We can conclude that the design and workmanship of the lens fully corresponds its price. This Chinese unnamed 10x0.25 achromat has well-made, coated optics housed in a mediocre body with no signs of light protection. When using the lens "as is" you can expect a lot of problems with image contrast, which is especially critical when doing microphotography. On the other hand, the simplicity of the design is definitely in the hands of those who are ready to disassemble the lens and correct factory defects.

Image quality

The unnamed 10x0.25 achromat has good image sharpness in the central region at full aperture, but chromatic aberration in the form of purple edgings are extremely strong. In terms of on-axis image quality, the lens is noticeably inferior to a more expensive one 10×0.25 Plan and similar to the Soviet 8×0.2, 9×0.2 Plan or 9×0.2 Epi.

Unexpectedly, this 10×0.25 has an almost flat field, especially when you compare the lens with the Soviet one achromat 9×0.2 Epi. However, the residual astigmatism of this lens is more pronounced, and therefore it will not be possible to obtain images along the edges as sharp as in the center of the frame even when refocusing.

 

Judging by the black level, it is easy to see that the modified Chinese lens has even greater image contrast than the Soviet Epi 9×0.2, which, in general, I did not blame for the contrast. The reason for this is obviously the presence of an anti-reflective coating on the optics and the modification of the lens light protection. Moreover, it is worth noting: it is unlikely that the lens would have performed well without blackening - no coating of the optics will protect against re-reflections of light in the long tube between the lenses!

The Chinese lens significantly improves in terms of field resolution and when compared with the modified one LOMO 10×0.4 L (with 8 mm aperture) – due to a lower level of astigmatism and near-zero lateral chromatism. But in the central region, the Chinese lens is noticeably inferior to it.

 

In this regard, an attempt was made to limit the lens aperture to level out spherochromatism - next to the standard aperture with a diameter of 9 mm, another one with a diameter of 6 mm was installed. Thus, the lens has lost approximately 1 stop of aperture, which corresponds to a numerical aperture of ~0.18 and aperture of ~F/2.8. With these parameters, the diffraction limitation of resolution is 40 lines/mm versus 55 lines/mm with a numerical aperture of 0.25. Is this loss significant? Not at all: the results of modeling the optical design of the LOMO Plan 9×0.2 lens, which is similar in image quality on the axis to the Chinese 10×0.25 in question, showed that its real resolution does not exceed 30 lines/mm due to the influence of aberrations when used on a modern camera. Therefore, you don’t have to experience any torment when stopping down the Chinese 10x0.25 from 9 to 6 mm.

Aperture really brings benefits - with an aperture of 6 mm, the magnitude of longitudinal chromatic aberrations turns out to be noticeably lower, although it was not possible to outperform the modified LOMO 10x0.4 L in image quality in the central region. Perhaps if the Chinese had a “lead” version of this 10x0.25, the result would have been better.

 

Below are photos without stacking on a full frame camera Sony A7s and a modified 10×0.25 lens – with blackening and a 6 mm aperture – on a modified NPZ M-10 microscope with a tube distance of 160 mm.

List of objects in the photo: 1 – Letter from the LOMO emblem on the micrometer object, 2-4 – Crystals of chromium(III) acetylacetonate, 5-6 – crystals of elemental sulfur, 7-8 – crystals of potassium bisoxalatocuprate hydrate, 9-10 – crystalline sulfide -zirconium disulfide, 11 – needle file surface, 12 – plant leaf hairs in polarized light.

Next are photographs taken with stacking in Helicon Focus. I recommend paying attention to this program for those who are interested in macro photo processing: unlike stacking using standard Photoshop tools, in Helicon Focus image processing occurs in multi-threaded mode (several times faster on multi-core processors), more controlled and adequately even with standard settings.

List of objects: 1-3 – crystals of chromium(III) acetylacetonate, 4-6 – crystals of elemental sulfur, 7-8 – crystals of potassium bisoxalate cuprate hydrate, 9-11 – crystalline zirconium sulfide-disulfide, 12 – needle file surface.

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

Modern optics from Chinese manufacturers:

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

Reviews of Soviet lenses for microscopes:

• LOMO Epi 9x0.2 (adapted)
• LOMO 10x0.4 L (OM-33L) - modification and test
• Review of achromat microscope lens LOMO 21×0.4 190-P (OM-8P)

Conclusions

Simple modern unnamed 10×0.25 – a good budget solution that fully justifies its cost. The lens is not without its drawbacks - the simple optical design does not control spherochromatism well, the extreme reduction in cost has led to the loss of light protection and, as a consequence, the loss of image contrast when using “as is”. However, the shortcomings of the lens are quite correctable, and, after a simple modification, the lens can definitely fully replace all sorts of old cheap 8-10x achromats. It is very likely that a fundamental improvement in the quality of a 10x lens can only be achieved by greatly complicating its design and using expensive materials, which means that this no-name 10×0.25 can be considered “the best among the worst,” including even among some more expensive lenses.