Material on the lens especially for Radozhiva prepared Rodion Eshmakov.
LOMO 21×0.4 190-P in the revolver of the NPZ M-10 microscope.
Objectives with a magnification of 20x, as a rule, are not included in the standard equipment of modern microscopes, and therefore many microscopists are puzzled by the search for an intermediate objective between 10x and 40x: a 20x lens allows you to see more detail than a 10x, but it is also easier to work with than a 40x.
The medium magnification achromat lens LOMO 21×0.4 190-P (OM-8M) presented in this review was included in the set of old Soviet polarizing microscopes. For such lenses, optics sets were specially selected so that there were no mechanical stresses in the lenses. The 21×0.4 objective is mechanically compatible with RMS end-tube microscopes, and is designed for a tube distance of 190 mm. There is also a regular version of the lens for a 160 mm tube – LOMO 20×0.4 (OM-27), and modern analogue This lens is made in China.
Technical specifications
Optical design - 5 lenses in 3 groups;
Drawing of the optical scheme of the lens.
Type of correction – achromat;
Tube distance – 190 mm;
Magnification factor – 21x;
NA – 0.4 (requires illumination condenser or lens illumination for optimal performance);
Focal length – ~8 mm;
Working distance – ~1.5 mm;
Cover glass thickness – 0 mm (designed for use without glass, version 20×0.4 – with 0.17 mm glass);
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 21×0.4 190-P lens is made in a very compact short body made of chrome-plated brass. The optical design of the lens is placed end-to-end in the body. The objective lenses are enclosed in metal frames, and the second element is additionally centered through 4 holes in the body at the factory during assembly. It is better not to disassemble this lens unless absolutely necessary - it will be difficult to put it back together correctly and adjustment will be required.
LOMO 21×0.4 190-P can be mounted on a regular microscope of the RMS standard with a 160 mm tube, but the magnification will be slightly less than stated, and field curvature and astigmatism may increase. The main problem with this use is the length of the lens and its short working distance (about 1.5 mm) - when focusing, other lenses in the revolver get in the way, resting against the table. The nose of the lens is not spring-loaded, so it is important when focusing to monitor the distance to the object so as not to accidentally crush it and the unprotected front lens of the lens.
When using a 21x0.4 objective on a conventional microscope, other objectives in the revolver may interfere.
Exist special extension rings, which will allow you to use any lens with a tube distance greater than 160 mm without interference.
Unlike the regular version of LOMO 20×0.4 160/0.17, this lens has coated optics, which should have a positive effect on image contrast. The lenses are coated in a single layer, most likely applied physically. The short-wavelength limit of light transmission is at ~330 nm.
Light transmission spectrum LOMO 21×0.4 190-P (OM-8P).
It is also worth noting that in the USSR they neglected the light protection of lenses to a lesser extent than in modern Chinese production: everything inside the lens block looks dark, although not perfectly frosted.
The optical design of the lens was designed, apparently, by Ernst Abbe and is ideologically similar to aplanatus Richter with an additional thick collecting lens (“Herzberger converter”). According to X-ray fluorescence spectroscopy, the front lens of the lens is made of lead flint (probably heavy flint of the TF brand). The spectrum of the rear lens, moreover, turned out to be identical, but, taking into account the type of circuit, this only means that the inner gluing lens is made of the same flint, and the outer one, on the contrary, does not contain heavy elements - that is, it is crown or light crown.
X-ray fluorescence spectrum of the front lens of the LOMO 21×0.4 190-P objective. Detection of Cr, Cu – lens frame signal. Ar – air. Zr is an instrumental artifact. K, Zn, Pb, As were found in glass.
X-ray fluorescence spectrum of the rear bonding of the LOMO 21×0.4 190-P lens. Actually refers to the internal bonding lens. Detection of Cr, Cu – lens frame signal. Ar – air. Zr is an instrumental artifact. K, Zn, Pb, As were found in glass.
Photos of the lens's appearance are shown below.
For the most part, the LOMO 21×0.4 190-P gives the impression of an old, but relatively high-quality lens, made a little better in comparison with the most popular Soviet 20×0.4 160/0.17 achromats. The biggest disadvantage when used on microscopes with a tube distance of 160 mm is the need to compensate for the working distance or put up with the inconvenience of focusing with other objectives in the revolver.
Image quality
LOMO 21×0.4 already has a sufficiently large numerical aperture of 0.4 to make it necessary to use a condenser for illumination. The point is not that otherwise there will be a lack of light, but that the condenser allows you to direct the light at an angle to the object, forming a cone of illumination, thereby increasing the contrast and resolution of the image. This means that the lens is not recommended for reflected light photography with side lighting, which is already difficult to achieve due to the short working distance. The ideal solution would be illumination directly through the lens, implemented in some microscopes or using special attachments. Almost any standard condenser is suitable for observations in transmitted light. If the microscope is not equipped with it, the image quality will be severely limited.
In transmitted light, when using a condenser, the lens shows a picture with a large number of spherochromatic aberrations (purple-yellow fringing, unclear contours). As you move away from the center of the frame, the image deteriorates greatly due to the influence of field curvature, astigmatism and lateral chromatism. When refocusing, it is impossible to achieve sharpness at the edges of the field. Perhaps, when compensating for the tube distance, the quality across the field will be slightly better, but lateral chromatism will not help correct this.
In reflected light, the image quality is significantly reduced, the picture appears loose and grainy, apparently due to some kind of wave effects.
Image of the LOMO OMO-U4.2 reflected light object micrometer, taken on a Sony A7s and a LOMO 21×0.4 lens with a tube length of ~160 mm. The length of the mark is 1 mm, the division value is 0.01 mm.
The depth of field when using such a lens is very shallow, in most cases stacking is very desirable.
Despite all the shortcomings, the lens is quite usable if nothing better is at hand. First of all, when shooting with this lens, you need to pay attention to lighting - the result largely depends on it.
The following are examples of photographs taken with a full-frame mirrorless camera. Sony A7s and a LOMO 21×0.4 190-P lens (without tube distance compensation), mounted on a modified NPZ M-10 microscope, without the use of stacking.
List of objects in the photo: 1-2 – potassium oxalatocuprate hydrate, 3-4 – crystalline film of sulfur obtained by crystallization of a solution of sulfur in cyclohexane on glass; 5-7 – crystalline film of sulfur obtained by crystallization of a solution of sulfur in toluene on glass; 8-9 – fine-crystalline barium(VI) manganate on the surface of graphite, 10-11 – chromium(III) acetylacetonate, 12 – intergrowths of manganese(II) sulfate monohydrate crystals.
The process of crystallization of sulfur from a solution in toluene
Below are examples of photos using stacking.
List of objects in the photo: 1 – polymer chromium(II)-hydrazinium sulfate, 2-3 – potassium oxalatocuprate hydrate, 4-5 – crystalline sulfur film obtained by crystallization of a solution of sulfur in toluene on glass; 6 – finely crystalline barium manganate(VI) on the surface of graphite, 7-8 – chromium(III) acetylacetonate, 9 – intergrowths of manganese(II) sulfate monohydrate crystals, 10 – potassium hexanitron nickelate(II).
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 and comparative test of microscope achromat 20/0.40 160/0.17 (China, no-name)
- 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
LOMO 21×0.4 190-P is an old, optically very mediocre microscope lens that is poorly suited for shooting at direct focus due to a high level of lateral chromatic aberrations and a generally high level of chromatism. Meanwhile, 20x magnification is very convenient for a number of applications, and if it is necessary to equip a microscope with additional lenses, it is better to pay attention to modern budget Chinese options and more expensive planchromats.
