Review of the high-aperture lens for the LOMO 10x0.4 L microscope (OM-33L)

Material especially for Radozhiva prepared Rodion Eshmakov.

The LOMO 10×0.4 L lens is a high-aperture, low-magnification achromat designed for equipping fluorescent microscopes (letter L in the marking). The lens is fully compatible with conventional microscopes of the RMS standard with a tube distance of 160 mm, however, there are a number of nuances when using it, which will be discussed in this article. About what microscope optics is, how to use it for photography and what language they speak about it, written here.

Specifications:

Optical design – 7 lenses in 4 groups, does not use special glasses in the optical design;

Type of correction – achromat;
Tube distance – 160 mm;
Magnification factor – 10x;
Numerical aperture – 0.4;
Lateral chromatism (increase chromatism) – 1%;
Focal length (tube length ÷ magnification) – 16 mm;
Relative aperture – ~F/1.2;
The estimated image field size is 18 mm;
Parfocal distance – 45 mm;
Working distance – 3.08 mm;
Cover glass – 0.17 mm, K14 (H-K7);
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

LOMO 10×0.4 L is made of a shiny chrome-plated brass body and is visually similar to fairly large high-magnification microlenses. Unlike the usual small standard low-magnification achromats such as LOMO 8×0.2 or 10×0.25, this one has a removable body jacket, under which you can find 4 holes for adjustment. The essence of this process is to center the second lens component of the lens relative to the others by displacing it in the assembled lens using needles through holes in the body, while it is additionally possible to independently rotate the front and last lens components. Ideally, the lens should be adjusted at the factory once, the holes should be filled with sealant and no one else should get in there. In harsh reality, a 1-year-old lens has probably already been tampered with, and the factory assembly could have been done quite in style Izyumsky plant, so the huge problem with this lens is that after purchase, it will most likely have to be adjusted, which is not easy, painstaking and requires some experience. My lens turned out to be problematic and I had to try to “bring it to normal operation,” which was only partially successful: while I was able to completely overcome the coma on the axis, a small astigmatism remained, but noticeable at full aperture.

Another disadvantage of the lens is related to its design, which includes 3 glued elements: there is information that the LOMO 10×0.4 L often suffers from sticking in the lenses, so it is important to carefully inspect it before purchasing.

The working distance of LOMO 10×0.4 L is significantly smaller compared to other 8-10x lenses and is 3 mm. For comparison: the working distance of LOMO Plan 9×0.2 is almost 14 mm! The working distance greatly influences the ease of use of the lens for observations and shooting in reflected light with side lighting. In fact, 3 mm is still acceptable, but in comparison with 9×0.2 Epi There are still some inconveniences.

A nice feature of LOMO 10×0.4 L is the presence of an antireflection coating on all glass-air surfaces, and the antireflection has a yellow highlight, and not a violet one - typical for optics for the visible spectrum. As it turns out, this choice of coating is due to the fact that the lens is used in systems where illumination with ultraviolet light is carried out directly through the lens - and it is the coating with a yellow highlight that gives maximum transmission in the violet part of the spectrum.

The short-wavelength transmission limit is in the region of 330 nm - apparently, the lens does not use heavy flint lenses to reduce self-luminescence and increase the transmittance of UV radiation. Strict restrictions on the choice of optical materials make the development of high-aperture (high-aperture) lenses a very difficult task, and therefore LOMO 10x0.4 L has as many as 7 lenses in the optical design (a typical achromat type 8x0.2 - 4 lenses). The question of optical quality, of course, remains - after all, the lens does not use low-dispersion glass (special, phosphate and heavy phosphate crowns), which are very useful for such optics. But even abstracting from the image quality, we can already draw a definite conclusion that the LOMO 10×0.4 L is a toy that is extremely “for everyone” due to the difficulty of finding a good copy, as well as the short working distance. If about quality LOMO Epi 9×0.2 While we can definitely say that after adaptation it will have a huge advantage over the usual 8×0.2 or LOMO Plan 9×0.2, the same cannot be said about this lens.

Optical properties

To be honest, the LOMO 10×0.4 L is probably the champion in terms of the number of aberrations among 10x lenses. With its huge numerical aperture of 0.4, the lens has the same huge level of spherochromatic aberrations, which is why, despite good resolution, the contour and overall contrast of the lens are simply terrible. The curvature of the image field, as well as astigmatism, are also terrible in their magnitude. The level of lateral chromatism is very high - the lens is designed for use with compensation eyepieces. Although, in comparison with many Soviet APO lenses, the chromaticity is still tolerable.

Of course, I initially had no illusions about optical quality, and therefore a possible solution to the problem was prepared in advance in the form of fixed-size aperture diaphragms for installation immediately after the rear lens of the lens. This positioning of the diaphragm is very typical for microscope optics. Using 3D printing, 3 apertures were made: 11.5 mm, 8 mm and 5.7 mm.

The main point of the manipulation is that if the aperture of a high-aperture lens with strong spherochromatism is limited, it is often possible to obtain better image quality in comparison with a simpler lens with the same aperture. In other words, having an old fix like Helios-81N 50/2 at F/4 we will get higher image quality compared to Industar-61 LZ 50/2.8 at the same aperture.

Testing by considering an object-micrometer and a real object showed that an 10 mm aperture is best suited for the LOMO 0.4x8 L, turning it into a ~10x0.25 lens. A larger aperture does not allow you to get rid of spherochromatism, a smaller one leads to a strong manifestation of diffraction. Diaphragming does not overcome lateral chromatism, but it can be partially corrected using software.

The following are photographs of a real object in transmitted light using different apertures and 100% cropping of the central area of ​​​​the images. exposure was leveled.

It is clearly visible how lens aperture has a beneficial effect on field sharpness and depth of field. We must not forget that with aperture, the resolution determined by the diffraction limit also decreases, but in the case of this lens with its mediocre assembly, you can be sure that it is not diffraction, but astigmatism that limits its resolution.

After all these observations, it became unclear whether there was the slightest point in torturing this lens and whether it had at least some advantage over the simple and understandable achromat LOMO 9×0.2 Epi. As it turned out, the aperture LOMO 10×0.4 L (D=8 mm) in comparison with the LOMO Epi 9×0.2 turns out to be significantly better in the central region of the field in terms of resolution and the level of spherochromatic aberrations, which means that colored fringe in the case of using this problematic lens will have much less impact on the image, especially when stacking.

As for field aberrations, a fast lens is inferior both in terms of field curvature and astigmatism, and in terms of chromatic aberrations. Nevertheless, the lens is at least no worse in lateral chromaticity than the new Chinese Plan 10×0.25 (from this https://radojuva.com/2024/05/plan-4-x-0-1-micro/ series). From the point of view of choosing the size of the working field, the best option is to crop the resulting images to a 7:6 or 4:3 format, taking into account the existing vignette.

A very significant fact in favor of the LOMO 10×0.4 L (with an 8 mm aperture) is the good overall contrast of the image formed. Despite the larger number of lens elements in the optical design, the lens behaves absolutely no worse than the LOMO Epi 9×0.2. The anti-reflective coating of the lenses and the absence of obvious problems with light protection did their job.

The following are examples of photos taken with a full-frame Sony A7s camera and a LOMO 10x0.4 L lens (with an aperture of 8 mm) without using stacking. Some of the pictures were cropped. Description of objects in the photo: 1 – Hexaamminnickel(II) chloride, octahedral crystals; 2 – elemental sulfur crystals obtained from a solution in cyclohexane; 3-5 – crystals of elemental sulfur obtained by evaporating a drop of a solution of sulfur in toluene on glass; 6 – crystals of elemental sulfur obtained by evaporating a drop of a solution of sulfur in toluene on glass, crossed polarizers; 7 – elemental sulfur crystals obtained from a solution in chloroform; 8 – potassium bisoxalatocuprate hydrate; 9-10 – tin tetraacetate.

Then - examples of photos in the same conditions, but using stacking. Objects in the photo: 1 – hexaamminnickel(II) chloride, octahedral crystals; 2 – potassium biscoxalatocuprate hydrate, lamellar crystals; 3 – orthorhombic crystals of elemental sulfur obtained by evaporating a drop of a solution of sulfur in toluene on a glass plate; 4-5 – elemental sulfur crystals obtained from a solution in chloroform; 6-8 – elemental sulfur crystals obtained from a solution in cyclohexane (strong light dispersion); 9-10 – tin tetraacetate.

Bonus: 2 animated images demonstrating the depth of field of the LOMO 10x0.4L (8mm aperture) on the link here.

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

LOMO 10×0.4 L is a problematic lens with very compromised qualities. A successful specimen, only when stopped down, is capable of producing an image that is superior in certain parameters to the image from very cheap and affordable Soviet 8-10x lenses and, apparently, some modern cheap Chinese lenses. But still, the modification, maintenance and use of this lens is a lottery for enthusiasts. For its intended use - as a luminescent lens, for a narrow part of the spectrum - the lens is much better suited than for use as a regular low-magnification lens.

You will find more reviews from readers of Radozhiva here.