Material on the lens especially for Radozhiva prepared Rodion Eshmakov.
Plan 20×0.4 in the revolver of the NPZ M-10 microscope.
Lenses with 20x magnification occupy an intermediate position between typical standard 10x and 40x lenses, providing, on the one hand, relative ease of use compared to 40x lenses, and on the other hand, a higher level of detail compared to a 10x lens. This review presents a modern inexpensive lens 20×0.4 Plan for biological microscopes of the RMS standard with end tube. Some basic information regarding the use of microscopic optics on cameras is given here.
Technical specifications
Optical design – unknown;
Correction type – planchromat;
Tube distance – 160 mm;
Magnification factor – 20x;
NA – 0.4 (requires illumination condenser or lens illumination for optimal performance);
Focal length – ~8 mm;
Working distance – 0.65 mm;
Cover glass thickness – 0.17 mm (when used without glass, the image quality may be lower than calculated);
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
Planachromat 20×0.4 has a completely metal body. The lens is quite large and weighty. The removable decorative outer part of the lens appears to be made of aluminum, while the lens block body and aperture diaphragm are made of brass. The lens jacket is not fixed, so instead of the lens from the microscope socket, you can often twist it separately.
Plan 20×0.4 has a short working distance - less than 1 millimeter, which is quite inconvenient: there is a risk of ramming the sample when focusing with the nose of the lens, and it is almost impossible to provide side illumination. It’s good that the front lens of the lens is made concave and does not protrude beyond the body. To protect the drug and the lens from damage in the event of a collision, the lens block is spring-loaded.
The disadvantage of the Chinese planchromat is the low quality of blackening - in some places it is completely absent, as on the open matte surface of the front lens. It is not recommended to disassemble the lens block itself for blackening, since after this you will need to re-adjust the lens yourself, establishing the correct position of the frame with lenses on the optical axis through holes in the walls of the lens block. At the same time, the entire lens block can be removed and everything behind the rear lens can be matted to increase image contrast.
The lens optics are coated with purple shades, probably one- or two-layer. The short-wavelength limit of light transmission is ~350 nm.
Light transmission spectrum of the Plan 20×0.4 lens.
The lens is made using modern optical materials, in particular, highly refractive lanthanum flints. Thus, according to X-ray fluorescence spectroscopy (Bruker M1 Mistral), the front lens of the lens is made of heavy lanthanum flint with a refractive index of ~1.8 and an Abbe number of ~40.
X-ray fluorescence spectrum of the front lens of the objective lens. La, Y, Zr, Nb were found. Detection of Cr, Ni, Cu, Zn – glow of the lens frame.
Photos of the external lens are shown below. The lens also exists in another design option - in a chrome-plated brass body. When choosing, you should focus on the location, size and shape of the lenses.
With the exception of the traditionally low quality of light protection for inexpensive microscope optics, the lens does not have any significant shortcomings - it feels like a nice quality item.
Image quality
In order for the image formed by this lens to have maximum quality, the microscope must have an illumination condenser with sufficient numerical aperture. In the case of reflected light shooting, almost the only option is to use lighting through the lens, if the system allows it, since the working distance of Plan 20x0.4 is too small to supply a side illuminator.
Compared to the Soviet lens LOMO 21×0.4 190-P This Chinese Plan 20x0.4 provides significantly better image quality both in the center and in the field.
Image quality test of Plan 20×0.4 and the old Soviet LOMO 21×0.4 190-P. 100% cropped photographs of the eyepiece scale taken with a Sony A7s camera.
I used the lens without a cover glass and observed some residual spherochromatic aberration in the central area of the frame. Perhaps if glass is used, the result will be better.
Along the field, image quality is most reduced due to lateral chromatism, which is undercorrected in this lens. Curvature and astigmatism have a lesser effect. Most likely, for microscopes with a tube distance of 160 mm there is simply no line of modern Chinese optics with corrected magnification chromatism. If I’m wrong, write in the comments if you know of such lenses with near-zero lateral chromatic aberrations with a factor of more than 10x at the final tube distance. To some extent, edge chromaticity can be dealt with programmatically - for example, when developing RAW files in Adobe Camera Raw.
The lens has a shallow depth of field; in most cases, it is advisable to use stacking when shooting. Some samples, such as thin crystal films, can be shot without stacking frames.
The following are examples of photographs taken with a Plan 20/0.4 160/0.17 lens and a mirrorless full-frame camera Sony A7s, mounted on a modified microscope Oil Refinery M-10. All photographs were taken without the use of a cover glass.
List of objects in the photo: 1 – ammonium nitrocerrate(IV) hydrate, reflected light micrometer object LOMO OMO-U4.2, 3-4 – octahedral crystals of hexamine nickel chloride, 5 – sulfur crystal with screw dislocation, 6 – measuring eyepiece grid, 7 -10 – crystalline film of sulfur obtained by crystallization of a solution of sulfur in toluene on glass.
Then - examples using staking.
List of objects in the photo: 1-3 – octahedral crystals of hexaamminnickel chloride, 4 – chromium(III) acetylacetonate, 5 – zirconium sulfide-disulfide, 6 – plant leaf with visible cells and chloroplasts, 7-10 – crystalline sulfur film obtained by crystallization on glass of a solution of sulfur in toluene; 11 – violet vanadium(II) sulfate, which quickly oxidizes in air into a black oxo compound; 12-13 – potassium bisoxalatocuprate hydrate, 14 – ammonium nitrocerrate(IV) hydrate.
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
The Chinese Plan 20x0.4 160/0.17 lens will probably be a very good solution for older microscopes for visual observations using compensation eyepieces. Due to well-corrected aberrations in the central region of the field, the lens is more suitable for direct focus photography than old Soviet 20x lenses, but due to the presence of lateral chromatic aberrations it is far from ideal. A good argument for purchasing this lens is its low price.
