Microscope objectives 3.7x0.11 (OM-12), 4.7x0.11 (LOMO, Progress): review and test

Material on the lens especially for Radozhiva prepared Rodion Eshmakov.

Soviet achromatic lenses with the OM-12 code and parameters of 3.7×0.11 (for a final tube distance of 160 mm) and 4.7×0.11 (tube 190 mm) are currently considered (including among foreign photographers) to be some of the best budget solutions for macro photography. Their popularity is due to the optical properties of this lens, which are more in line with more expensive optics (some even call this lens an "apochromat", which, strictly speaking, is not true at all), as well as its versatility, prevalence and low price.

The 3.7×0.11, 4.7×0.11 objectives are based on a very old pre-war objective designed at the Russkie Samotsvety enterprise and were intended for a wide variety of microscopes with RMS-type mounts – the simplest Mir measuring microscopes, the MBI-1 research microscopes, the polarizing mineralogical microscopes, and the simplified Erudite educational microscopes.

Technical specifications

Source – reference book “Computational Optics”, ed. MM. Rusinova, L., 1984, p. 336.
Optical design - 3 lenses in 2 groups, aplanat;

Type of correction – achromat;
Tube distance – 160 mm (option 3.7×0.11), 190 mm (option 4.7×0.11), the lens works satisfactorily with an infinity tube;
Parfocal distance – 50 mm (option 3.7×0.11);
Magnification: 3.7x (160 mm tube), 4.7x (190 mm tube);
Numerical aperture – 0.11;
Focal length - 33.1 mm;
Working distance – 27.2 mm (option 3.7x0.11);
Cover glass thickness: 0-0.17 mm;
Chromatic difference of magnification – 0% (for lines F, C);
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 OM-12 type lenses have been produced for a long time and can be found in different external design variants. There are no surprises in their design: the lenses of the lens in autocollimation frames with an interlens insert are simply poured into a chromed brass body and fixed with a slotted nut on the back of the lens.

The main differences between the different versions of the OM-12 are in two points: the calculated tube distance (2 mm for the 160x version, 3.7 mm for the 190x version) and the presence of an anti-reflective coating on the lenses (present on later versions of the lenses with a “thick” body). Moreover, as shown below, if a lens marked 4.7×4.7 on a microscope with a tube distance of 0.11 mm behaves exactly the same as 160×3.7, then the image from the coated and uncoated lenses differs significantly more. According to the data of light transmission spectroscopy, the coating of the lenses corresponds to a single-layer coating (like vacuum MgF0.11), the absorption edge in the short-wave region corresponds to ~2 nm. Versions with an anti-reflective coating, apparently, have significantly better light transmission compared to uncoated versions.

The quality of light protection and blackening of the inner surfaces of the lens is quite mediocre in all versions.

It is important to note that the OM-12 objective has a non-standard parfocal distance of 50 mm - this is much more than for objectives of Soviet biological microscopes such as MBI-1/MBR-1/Biolam (standard 33 mm) and 5 mm more than the currently common standard of 45 mm. This means that when changing the objective to another one, refocusing will be required, sometimes very significant. Fortunately, the parfocal distance is greater, not less - when changing the objective, there is no risk of colliding with an object. The convenient large working distance of the objective of 27.2 mm also contributes to this.

According to X-ray fluorescence spectroscopy, the front element of the objective is more of a single lens than a double crown-flint bond: due to the very small thickness of the lenses, in the case of the presence of flint, regardless of the orientation of the bond, its characteristic signal (lead lines) is observed very clearly, which is not observed in this case.

The second component is a glued crown and lead flint.

It was not possible to establish the brands of optical glass due to the lack of reference samples. Probably, both positive lenses of the objective are made of zinc crown glass (brand K, BK) or barium crown glass (brand BK), and F or TF glass is used as a flint. In other words, no special (with special dispersion) glass is used in the objective.

Thus, it is most likely that the lens has a very simple three-element design, and not a classic four-element design like the lens M-42 8×0.2.

Image quality. Comparison with analogues

The 3.7×0.11 lens produces a sharp image in the central area of ​​the image with a pronounced field curvature. It is important that astigmatism is well corrected within the APS-C frame, so by refocusing it is easy to achieve a sharp image at the edge of the field. Comparatively small spherochromatic aberrations (purple-green fringe) can be seen on the contrasting borders.

The 3.7×0.11 lens version with anti-reflective coating has noticeably higher contrast and light transmission.

To assess the image quality, the following lenses were tested in reflected light using a LOMO OMO micrometer (division value 0.01 mm): Plan 4×0.1 160/- (China), LOMO OH-26 4×0.12 (without coating), LOMO 3.7×0.11 (with coating), Progress 3.7×0.11 (without coating), Progress 4.7×0.11 (with coating), Progress 4.7×0.11 (without coating) and two uncoated planachromats LOMO/Progress OM-3 3.5×0.1. The test was performed using a modified NPZ M-10 microscope with a tube distance of 160 mm and a camera Sony NEX-6.

Test photos taken with the LOMO 3.7×0.11 lens with coating, as well as 100% crops of the images are shown below.

Among the lenses tested, the OM-12 achromats, regardless of the design, demonstrate the lowest level of spherochromatic aberrations, and in terms of image field curvature, they are comparable to both the modern Chinese 4×0.1 planachromat (not such a “plan”) and the standard LOMO Mikmed-2/Bimam 4×0.12 microscope lens.

The OM-3 3.5×0.1 planachromat objective, made according to the “Tair” design, is ahead of all the tested objectives in terms of field curvature correction, but also has a high level of spherochromatism, coma, lateral chromatism, as well as worse overall contrast compared to OM-12 type objectives.

It is important to note that optically the 3.7×0.11 and 4.7×0.11 lenses are no different from each other.

Below are examples of photographs taken without stacking using a LOMO 3.7×0.11 lens (with coating) using modified MBI-1 and M-10 microscopes and Sony NEX-3 cameras. NEX-3Women, NEX-6 with APS-C matrix size.

List of objects in the photo: 1-3) Synthetic copper(I) telluride; 4-5) Synthetic copper(I) sulfide; 6) Synthetic bismuth sulfide; 7-14) Etched surface of bismuth selenide crystal with screw dislocation outcrops (triangles); 15-16) Iron(III) acetylacetonate; 17) Tungsten(VI) phenolate; 18) Copper(II) acetylacetonate.

Then – examples of photos using stacking, the same equipment.

List of objects in the photo: 1-7) Synthetic germanium-bismuth tellurides, 8-10) Synthetic copper(I) telluride; 11-12) Synthetic copper(I) sulfide; 13-17) Synthetic bismuth sulfide; 18-20) Etched surface of bismuth selenide crystal with screw dislocation outcrops (triangles); 21-26) Synthetic molybdenum(IV) telluride; 27-28) Iron(III) acetylacetonate; 29) Tungsten(VI) phenolate; 30) Copper(II) acetylacetonate.

Conclusions

The OM-12 3.7×0.11 and 4.7×0.11 lenses have proven their reputation as the best budget low-magnification micro lenses for photography. The coated versions have proven themselves especially well. The only real drawback of the lens is its non-standard parfocal distance. Overall, given the low price, I preferred this lens to the new one Chinese planachromat 4×0.1.

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)