Review and test of the OM-27 20x0.4 (Progress) achromatic microscope

Material on the lens especially for Radozhiva prepared Rodion Eshmakov.

The OM-27 20×0.4 objective is a simple achromatic micro objective that was standardly supplied with some Soviet school microscopes of the RMS standard with a final tube distance of 160 mm. The objective was additional for Soviet biological microscopes. There is also a modification of the objective for use with a phase contrast device (marked F), as well as a special version for polarizing microscopes with a tube distance of 190 mm – OM-8P 21×0.4. The OM-27 was produced by LOMO, Progress and LZOS. This article presents a lens produced by the Progress plant in a standard version.

Technical specifications

Optical design - 5 lenses in 3 groups;

Type of correction – achromat;
Tube distance – 160 mm;
Parfocal distance – 33 mm;
Magnification factor – 20x;
Numerical aperture – 0.4;
Focal length - 8.4 mm;
Working distance – ~2 mm;
Cover glass thickness – 0.17 mm;
Chromatic difference in magnification (distortion) – 8% (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 OM-27 lens is made in a chrome-plated brass body. The lens block is not spring-loaded: given the small working distance of ~2 mm, you should ensure that the observed object does not collide with the front lens of the lens when focusing. The decorative ring with marking is removable, under it there are 4 holes for adjusting the lens. It is better not to disassemble this lens unless necessary: ​​when the lenses become loose, they become decentered - then the lens will need to be adjusted so that it forms an image of adequate quality.

This version of the lens does not have an anti-reflective coating on the lenses. The quality of the internal blackening is also low - the surfaces are shiny.

Below are photographs of the appearance of the OM-27 lens, including a comparison with modern Chinese ones. achromat и planachromat with similar parameters.

In terms of performance, the OM-27 Progress is a typical Soviet cheap mass-produced micro lens. Poor blackening, no coating, simple metal body. The lens does not present any surprises.

Image Quality. Calculation Rating

The OM-27 lens produces an image with a low level of both general and contour contrast. Image detail is also quite mediocre. The lens has a curved field with pronounced astigmatism and lateral chromatic aberration. Already at the edges of the APS-C frame, the image is no longer focused.

Direct comparison with a modern Chinese analogue – unnamed achromat 20×0.4 – showed that the modern lens has a significantly higher overall and contour contrast of the image. In addition, the Chinese lens turned out to be approximately ~1/3-1/2 stops exposure lighter, which is probably due to higher light transmission. Below are examples of photos taken with the OM-27 lens and the Sony NEX-3 (APS-C).

Next are paired photos on a Chinese achromat 20×0.4 and image crops.

The OM-27 lens performed rather poorly, especially compared to a new Chinese lens of almost the same price, made using the same optical scheme of the Abbe achromat. Modeling the OM-27 optical scheme in Zemax showed that the lens has very pronounced spherochromatic aberrations and strong astigmatism, limiting the real working field to ~10-15 mm (for comparison, the diagonal size of an APS-C frame is 29 mm). The resolving power of the lens does not exceed 30 lines/mm when used with modern cameras, which is very little. In addition, the contrast level of 0.6 at a frequency of 10 lines/mm (contour contrast) is completely insufficient for comfortable photography - such bad photo lenses, for example, are hard to find, unless they are soft-focus lenses. The spot graphs, frequency-contrast characteristics and aberration diagrams are given below.

Below are examples of photos taken on a Sony NEX-3 using a modified MBI-1 microscope, condenser with NA=0.3.

Objects in the photo: 1) Potassium oxalatocuprate; 2) Moth wing scales; 3-4) Sulfur crystals, 5) Iron acetylacetonate, 6-7) Potassium thiocobaltate; 8) Ammonium peroxofluorotitanate; 9-10) Unstained blood smear.

Then – the same thing, but with stacking.

Objects in the photo: 1) Potassium oxalatocuprate, 2-3) Sulfur crystals; 4) Iron acetylacetonate, 5) Potassium thiocobaltate, 6) Ammonium peroxofluorotitanate.

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

Achromat OM-27 20×0.4 is an optically very weak lens. Its price on the secondary market is low, but given the availability of significantly more quality new Chinese lenses worth up to 20 $ the point in buying this Soviet 20×0.4 is minimal.