Review, analysis and large comparative test of microscope objectives LOMO Plan 9x0.20 and 10x0.20 (OM-2)

Material on the lens especially for Radozhiva prepared Rodion Eshmakov.

The LOMO Plan 9×0.2 and Plan 10×0.2 objectives are, despite the different markings, the same objective named OM-2, which in different versions was supplied with Soviet biological and polarizing microscopes with a final tube distance of 160 mm and an RMS mount. This review presents the later versions of the OM-2 objective from the standard set of the Biolam (Plan 9×0.2) and Mikmed-1 (Plan 10×0.2) microscopes, and also compares them with six other objectives with similar parameters: LOMO 10×0.4 L (OM-33L), LOMO 10×0.22 Plan L (OPH-10L), Chinese 10×0.25(0.2) achromat, LOMO Epi 9×0.2 (OE-9), Carl Zeiss Jena 10×0.3 and Progress 8×0.2 (M42).

Technical specifications

Optical design - 5 lenses in 3 groups;

Correction type – planchromat;
Tube distance – 160 mm;
Parfocal distance – 33 mm;
Magnification (actual) – 9.8x;
Numerical aperture – 0.2;
Focal length - 15.5 mm;
Working distance – 13.6 mm;
Cover glass thickness: 0-0.17 mm;
Chromatic difference in magnification (distortion) – 2% (400-700 nm);
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

The LOMO Plan 9×0.2 objective from the Biolam travel microscope kit was provided for review. Andrey Kulikov. The Plan 10×0.2 variant was received without factory accessories.

The lenses have identical design and body execution and differ only in marking. The body material is chrome-plated brass, there is not a single plastic part in the lenses. The marking is applied to the removable decorative outer part of the lens body by engraving. Unlike the common achromat 8×0.2, which can be found on a great many Soviet microscopes, LOMO Plan 9×0.2 has a more complex design with a front component centered through holes in the body. This means that when the ring fixing the lenses in the body is loosened, the lens becomes misaligned, which is important to know if you want to disassemble it.

The lenses of both objectives do not have any anti-reflective coating. The internal surfaces in the interlens space are noticeably shiny and have no ribbing. Due to the lack of anti-reflective coating, the transmission spectrum looks flat in the range of 500-1000 nm, but in the region up to 500 nm, absorption by the optical material is observed. The short-wavelength limit of light transmission corresponds to ~330 nm.

By today's standards, the lack of enlightenment and any light protection looks extremely bad. If high-quality blackening and ribbing on interlens inserts is a rare phenomenon for microscope optics in the budget segment, then even the most cheap lenses.
The optical materials used in the calculation are from the domestic catalog of the 1940s. Thus, the front and rear lenses of the objective are ordinary lead-containing glass. There are no glasses with special properties (highly refractive, low-dispersion) in the objective.

LOMO Plan 9×0.2 (10×0.2) has a very large working distance – 13.6 mm, which makes it very convenient for working with side lighting. The parfocal distance of the lens differs from the modern standard of 45 mm and is equal to 33 mm: when used with other, newer lenses (and some old ones), you will have to refocus.

The Plan 9×0.2 (10×0.2) objective was supposed to be an advanced replacement for the cheap 8×0.2 achromat. But in fact, the design has become more complex, the cost of the planachromat is clearly higher, and the quality of execution, if it shines, is only due to glossy interlens inserts and gray glare from uncoated optics.

Image Quality. Big Comparison

The image formed by LOMO Plan 9×0.2 and Plan 10×0.2 does not differ in anything, including the actual magnification value.

In the central area the image quality is at same level with other similar lenses like the 8×0.2 achromat. You can see pronounced spherochromatic aberrations. The image field within the APS-C frame is really flat with good astigmatism correction, but at the same time significant lateral chromatism is noticeable. However, it is less pronounced than for LOMO 10×0.4 L.

The key problem of the lens is the unsatisfactory image contrast, which is associated not only with the poor quality of the blackening of the inner surfaces, but also with the lack of an anti-reflective coating on the lenses. Of all the 8-10x lenses I know, the LOMO planachromat is the least contrasty.

A comparison of the image quality of the LOMO Plan 10×0.2 lenses was conducted, LOMO 10×0.4 L (OM-33L) , LOMO 10×0.22 Plan L (OPKh-10L, mediocre optics), Chinese 10×0.25(0.2) achromat, LOMO Epi 9×0.2 (OE-9), CZJ 10×0.3 and Progress 8×0.2 (M42) in transmitted light using a Sony NEX-3 (APS-C) camera mounted on a modified MBI-1 microscope.

During testing, 3 shots were taken for each lens: one with the focus in the center, the second for an object located at the edge of the APS-C field and with the same focus, the third with refocusing on the object at the edge of the field. Below are the corresponding photos for the LOMO Plan 10×0.2 lens, as well as crops of photos from the above lenses.

Based on the testing results, it can be noted that the most balanced characteristics are apparently possessed by the LOMO Plan 10×0.22 L (OPKh-10L) lens, which, despite its poor condition, demonstrates good detailing and a fairly even field with satisfactory astigmatism correction and no lateral chromatic aberrations. The main disadvantages of this lens are its rarity and extremely high price on the secondary market.

Next on the list of leaders is Chinese lens 10×0.25, modified with blackening and diaphragming to a numerical aperture of 0.2: among those tested, it has the best image contrast, a low level of longitudinal chromatic aberration, an almost flat field with no lateral chromatic aberration. Of the obvious problems, there is a fairly high level of astigmatism. But the fact that this lens can be buy for a modest price in any quantity, and the modification is quite simple, making it very attractive.

The 10×0.3 160/- lens (presumably made by Carl Zeiss Jena) stands out among the tested ones for its image detail in the center of the frame, but also has the greatest field curvature, as well as pronounced lateral chromatic aberration. In terms of image contrast, the lens is not much inferior to the modified Chinese one. The lens has similar properties LOMO 10×0.4 L with aperture D=8 mm (NA~0.25), but its overall contrast is noticeably lower, as is the degree of astigmatism correction.

Lenses LOMO Epi 9×0.2 and Progress 8×0.2 differ fundamentally only in light protection and magnification: due to the presence of enlightenment and ribbing of the internal surfaces, Epi 9×0.2 has much higher contrast than 8×0.2. At the same time, Epi 9×0.2 is noticeably inferior in image quality in the center to the modified Chinese achromat, has a greater curvature of the field, but a lower level of astigmatism.

In this comparison, the only advantages of the LOMO Plan 10×0.2 are its flat field and corrected astigmatism, but otherwise the lens is more of an outsider.

Below are examples of photographs without stacking, taken with a LOMO Plan 10×0.2 and a Sony NEX-3 (APS-C) camera mounted on a modified MBI-1 microscope.

List of objects in the photo: 1) hexaamminenickel(II) chloride octahedra, 2) flat copper(II) acetylacetonate needles, 3) nickel schönite intergrowth, 4) iron(III) acetylacetonate needle intergrowth, 5) potassium oxalatocuprate plates, 6) dead moth eye, 7) and 8) moth wing scales, 9)-11) rhombic sulfur, 12) potassium thiocyanatocobaltate, 13) coin fragment, 14) blade edge, 15) smartphone IPS display.

Then – images using stacking.

List of objects in the photo: 1) octahedra of hexaamminenickel(II) chloride, 2) flat needles of copper(II) acetylacetonate, 3) intergrowth of nickel schenite, 4) intergrowth of needles of iron(III) acetylacetonate, 5) plates of potassium oxalatocuprate, 6) eye of a dead moth, 7)-9) rhombic sulfur, 10) potassium thiocyanatocobaltate, 11) fragment of a coin, 12) edge of a blade.

The Problem of Calculating a Quality 10x Lens

LOMO Plan 9×0.2, if we do not take into account the image contrast, is not and is not considered the worst objective in the class of budget microscope optics. However, the analysis of its optical characteristics using the modeling of the optical scheme gives a very depressing result.

Thus, when used with modern cameras, the lens is capable of providing a resolution of no more than 33 lines/mm - and this is within a circle of about 8 mm in diameter. The lens suffers from monstrous spherochromatism - the full size of the scattering spot even on the axis is about 400 microns, which leads to an MTF value for 10 lines/mm of only 0.6 - it would be very difficult to find a photo lens with such terrible contour sharpness! The field curvature in the lens is undercorrected, lateral chromatic aberration is at a level unacceptable for modern cameras. The optical quality of the LOMO Plan 9×0.2 as a photo lens is monstrous. However, as with most microscope lenses, as shown in the test above. What will need to be done to radically improve the image quality of a lens with a magnification of 10x?

First, it is necessary to correct spherochromatic aberration. This requires an increase in the number of optical elements (reducing the influence of spherical aberration) and the use of modern glasses from the fluorophosphate and heavy phosphate crown series (reducing chromatism). The use of highly refractive glasses allows for effective control of field curvature and astigmatism.

I have designed new objectives based on the LOMO Plan 9×0.2 optical scheme using the CDGM catalog: a seven-lens 10×0.22 “Fluor-Plan” class (planachromat with a reduced secondary spectrum) and a nine-lens planapochromat 10×0.25. The quality of the latter is diffraction-limited for the central area of ​​the image. The objectives are designed for use with matrices up to 36×24 format and have corrected chromatic aberration for the range of 400-700 nm, including lateral ones.

As you can see, only the 10×0.25 lens, an apochromat for wavelengths of 400-700 nm, has optical quality that is truly sufficient for use with modern cameras.

Unfortunately, the development, production and assembly of optics of this level is an expensive process, and microscopy is an extremely conservative industry, and therefore more or less high-quality optics, even at the level of LOMO Plan 10×0.22 (which is not even Fluor-Plan and has the same ~30-35 lp/mm resolution) are very rare, and most are content with cheap lenses made using optical schemes that are almost a century and a half old.

All reviews of RMS standard microscope objectives with a finite tube distance (160-190 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. Progress 9×0.20 190-P (OM-13P)
5. LOMO Epi 9x0.2 (OE-9, adapted)
6. LOMO 10x0.4 L (OM-33L) - modification and test
7. Review and test of the OM-27 20x0.4 (Progress) achromatic microscope
8. Review of achromat microscope lens LOMO 21×0.4 190-P (OM-8P)

Carl Zeiss lenses:

1. Carl Zeiss Jena Semiplan 3.2/0.10 160/- (DIN)
2. Carl Zeiss Jena 10/0.30 160/-
3. Carl Zeiss Jena 40/0,65 160/0,17 (DIN)

Lenses from other manufacturers:

1. Lambda 10/0,25 160/-

Conclusions

LOMO Plan 9×0.20 (10×0.20) is an ultra-budget solution from among old Soviet optics. The lens has a convenient working distance, a flat field and is probably good for visual use, but due to lateral chromatic aberration and very low contrast it is poorly suited for photography. Given the availability of more successful Soviet and Chinese lenses There is no point in looking for this planachromat specifically, but if you get it by chance, it is worth trying if there is nothing better.