Microscopic optics on a camera. Review of microscope lens Plan 4x0.1 160/0.17 (China, no-name)

Material on the lens especially for Radozhiva prepared Rodion Eshmakov.

Lens Plan 4×0.1 160/0.17 – modern ultra-budget (~ 10 $) low-magnification (review) lens for the simplest biological microscopes of the RMS (Royal Microscopical Society) standard, made according to the classical design of the early 160th century (with a “final” tube of XNUMX mm). Lenses of this type can be used as fixed-scale macro lenses with digital cameras in cases where conventional photographic lenses with additional accessories do not provide the required image quality.

Specifications:

Optical design – not provided by the manufacturer; probably based on G. Taylor's "Triplet";
Type of correction – plan-achromat;
Tube distance – 160 mm;
Magnification factor – 4x;
Numerical aperture (NA) – 0.1;
Focal length (tube length ÷ magnification) – 40 mm;
Effective relative aperture (1 ÷ 2 NA) – f/5;
Parfocal distance – 45 mm;
Working distance – 29 mm;
Cover glass thickness – 0.17 mm;
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.

About the structure of a classical microscope

The history of modern optics, one might say, began with the improvement of microscopic lenses: the work Ernst Abbe, one of the founding fathers of optics as a science, was initially aimed specifically at creating high-quality microscopes for the company Carl Zeiss, as well as the achievements of a chemical technologist Otto Schotta, which gave rise to modern glass melting and optical materials science.

Microscopes with a finite tube distance have become widespread since the end of the 19th century and were replaced by systems with an “infinite” tube in the second half of the 20th century only in those areas where modularity of the optical system is required for use with various attachments: polarizing, luminescent and others. Today, such microscopes, being very simple to manufacture, are used mainly as educational or children's microscopes, although in the middle of the 20th century there were also complex research microscopes built according to the design with a finite tube. A microscope lens of this type forms an image of an object at a finite distance without the help of any additional optical elements (tube lens - in microscopes with an “infinite” tube). A camera matrix (for shooting at direct focus) or an eyepiece (for visual observation) can be placed in the image plane. There are also devices for photographing through an eyepiece, which makes some sense in some cases, but this approach is not considered within the scope of this article.

The distance from the image plane to the lens determines the magnification (the greater the distance, the greater the magnification), but to achieve an acceptable level of quality, the deviation of this distance from the calculated one should not be too large (±1-2 cm for magnification lenses <10x). Why is that? Just take a closer look at the schematic diagram of the microscope. It is not difficult to notice a certain similarity between this picture and, say, a picture with some typical long-focus lens focused at a finite distance - only the object and the image have already swapped places.

It is a well-known fact: ordinary photographic lenses are adjusted during calculation in such a way that their quality is best when focusing at infinity. Without the use of special design techniques, image quality inevitably drops when focusing at short distances, that is, the performance of a conventional photographic lens is affected by the image scale at which it is used. The same is true with micro-optics: it should be used at the scale for which it was calculated, that is, at the calculated tube distance - the distance from the image plane to the thread for attaching the lens to the microscope. The vast majority of lenses with an end tube are made at a distance of 160 or 190 mm.

The parfocal distance of a microscope objective is the distance from the mount to the object. This parameter is not so important for the user as for the designer: after all, it is the parfocal distance that determines how long the optical design of the lens can be. Limitations on optical design dimensions have a significant impact on the image quality of the highest-performance high-magnification, high-numerical aperture (NA) lenses. The simplest microscopes have a parfocal lens distance of 45 mm, while more complex research-grade systems have 65 or even 80 mm.

What matters is the working distance of the lens - that is, the distance from the front of the body to the object. This determines the ease of use of the lens: if the working distance is large, then it is easier to provide illumination of the object and there is less chance of damaging the front lens of the lens. Instrumental, metallographic and some other specialized lenses have the largest working distances, while biological ones usually, on the contrary, have short working distances (at a magnification of more than 10x), since with their help they examine only flat objects, and, as a rule, under a cover glass , the thickness of which is also included in the calculation of the lens, since the cover glass affects the correction of field aberrations. However, for low magnification lenses (up to 10x) this is usually not so important.

The size of the image formed is usually limited by the diameter of the microscope ocular seat. For the end-tube systems in question, this is a smooth 23.2 mm fit, so when shooting at direct focus through a microscope, vignetting can be observed even with APS-C format matrices.
Microscope optics are classified according to the type of correction of chromatic aberrations and field curvature. There are achromats, semi-apochromats (using fluorite-like materials) and apochromats (correction of chromatism in a wide range of wavelengths) - according to the type of chromatism correction, which can, in turn, be with corrected field curvature - that is, plan lenses.

Lenses for microscopes, regardless of the tube distance, are one-, two-, or three-component systems. The first component, which may be missing from low power lenses, is a powerful converging system, usually in the form of a hemispherical lens, either single or multiple. This component is no different in meaning from the speed booster in photographic lenses. The second component of the lens provides correction of aberrations, in some cases it is the only one. Very often this component can be designed as a lens Petzval or Richter, but “double Gaussian” (“Planar”) type schemes are also used. The third component, usually present in medium and high magnification plan lenses, is a field curvature and astigmatism corrector, and is often made in the shape of a meniscus, like lenses Maksutova or lenses Tair.

The optical designs of high magnification lenses are reminiscent of ultra-fast lenses such as Carl Zeiss R-Biotar, Astro Berlin Tachon and others - with incredible aperture values ​​of ~F/1.0 and a short rear segment. Lenses of small and medium magnifications are more similar in design to conventional film projection optics, for example - a lens KO-90 90 / 1.9 (Soviet LOMO 8×0.2, 9×0.2). The lowest magnification lenses (up to 4x) can be built like typical simple standard photographic lenses Triplet, Tessar etc.

The key problem when searching for microscope lenses for direct focus photography is the principle of mutual compensation of the lens and eyepiece, which was widely used in older microscopes with a finite tube distance to simplify the design of both eyepieces and objectives, especially at high magnifications. There is a very small list of old lenses suitable for use with a camera at direct focus (that is, without compensating optical systems, special eyepieces), and, as a rule, these are lenses with magnification up to 10x. Among them, for example, are LOMO Plan 3.5×0.1 lenses, Achromat 3.7×0.11, Achromat 8×0.2 (the most common microscope lens in general), Plan 9×0.2. The vast majority of Apo, Plan-Apo, 20x and larger lenses are designed for use with a compensation eyepiece and at direct focus have a very high level of spherochromatism, field aberrations and lateral chromatism. The absence of any systematic information about the design and parameters of microscope lenses makes the search for modern alternatives to old optics especially urgent - after all, new lenses are no longer considered to be equivalent to old compensation systems. However, not everything is so simple with Chinese optics: in the same body and under the same name there can be lenses with completely different quality levels. One way or another, Plan 4x0.1 160/0.17 is an alternative to the old Soviet LOMO 3.7x0.11 and Plan 3.5x0.1, which can be purchased completely new, not used, and often cheaper.

Plan 4×0.1 lens design and adaptation for cameras

The lens is made in a metal body, consisting of a removable aluminum jacket, which acts as a hood and forms the appearance of the lens, and, in fact, a lens block made of chrome-plated brass. The disadvantage of the design is that the outer body part does not have any blackening, and, as you know, a shiny lens hood is of little use. Fortunately, this can be easily fixed with a brush and black paint. Another problem is the lack of fixation of the lens jacket on the body. When unscrewing the lens from a microscope, the shirt is often unscrewed first, and then you have to grab the lens itself. Important: the lens comes in two external versions: with an anodized aluminum jacket (presented in this article) and with a chrome-plated brass jacket. Optically these are the same lens. Also important: there is evidence of the existence of Plan 4×0.1 160/0.17 lenses that are similar in appearance but optically different (for the worse).

Each of the optical surfaces of the lens has a purple anti-reflective coating. Not everyone is old Soviet lenses have enlightenment, and if they do, it’s extremely rare that the first lens does too. The blackening of the space inside the lens block between the lenses is quite mediocre.

According to X-ray fluorescence analysis, the front lens of the lens is made of lanthanum flint type glass (n ~ 1.75-1.8, v ~ 50-45): peaks of lanthanum, gadolinium, yttrium, niobium, and zinc are detected in the spectrum. Zirconium in the spectra is an instrumental artifact.

The rear lens element is made of heavy crown glass (n ~ 1.59-1.64, v ~ 61 – 57), as evidenced by barium, strontium and zinc peaks.

It is likely that if I took the lens apart, the middle lens or group of lenses would contain heavy flint, since there must be at least one high dispersion lens in the design.

As you can see, the cheap 4x0.1 lens is made using fairly modern materials, from which, for example, lanthanum flints were not available in the USSR. In other words, this lens is made at a higher technological level than its Soviet counterparts.

The appearance of Plan 4×0.1 160/0.17 is shown in the photo below. The lens is very compact in size, much smaller than any photographic one.

To use the lens with modern cameras, you can either purchase an adapter like RMS-M42 with a set of macro rings to achieve the required length of the tube, or, what is more convenient, convert some inexpensive microscope for photography. So, I got an NPZ M10 microscope from the 1940s-1950s, which was unclaimed in the laboratory, in which the standard eyepiece fit with a retractable tube was replaced by me with a fixed unit, which allows the microscope to be used for visual observations through wide-field eyepieces from MBS stereomicroscopes (fitting 32 mm ) and telescopes (1.25” fit), and for direct focus photography. In the same way, I remade the Soviet Biolam S11 for use in a training workshop.

When using a converted microscope, it is convenient to have a focusing mechanism, a stage and a transmitted light illuminator. By the way, light for microphotography is almost the most important thing. I used powerful LED two-point illuminator like this to provide lighting in reflected, transmitted or simultaneously reflected and transmitted light.

Optical properties

The Plan 4×0.1 lens demonstrates good image quality: spherical aberrations are well corrected in the central region (though due to mutual compensation) and the resolution is limited only by chromaticity; across the field, even beyond the calculated curvature and astigmatism are small, well corrected and lateral chromatic aberration. It is very difficult to find fault with the quality of a lens that costs $10 when alternatives, even if they are not always of better quality, are more expensive.

The lens performs well even with a tube length of up to 200 mm - in this case, the magnification becomes larger, as does the frame covered.

The contrast of the formed image strongly depends on the quality of the blackening of the tube. The blackening of the lens hood also has an effect, of course. After the required modifications, Plan 4×0.1 demonstrates a fairly good level of contrast and practically does not veil in transmitted light.

The light transmission of the lens also does not raise any questions: the antireflective coating (apparently one- or two-layer) only slightly reflects the red and violet regions of the spectrum, providing maximum transmission in the green region. The short-wavelength transmission limit of the lens is 350 nm.

The following are examples of photographs taken on Plan 4×0.1 160/0.17 using an M10 microscope and a Sony A7s camera with a tube length from 160 to 200 mm. I used the lens to photograph crystals of compounds obtained in a training workshop by 1st year students of the Faculty of Chemistry and others. For those interested, an indication of the substances: 1) ammonium molybdochromate(III), 2) piezochromic cobalt(II) molybdate, 3) luminescent cubane complex Cu4I4(C5H5N)4, 4) solvate of iron(III) acetylacetonate with chloroform in an ampoule, 5) and 6) potassium trisoxalatochromate trihydrate, 7) intergrowth of manganese(III) acetylacetonate crystals, 8) and 9) vanadyl acetalacetonate, 10) zirconium sulfide-disulfide.

Of course, the depth of field even with such a lens is quite small and stacking in some cases becomes a good solution. Below are examples of photos made using stacking in Photoshop. Indication of compounds: 1) ammonium molybdochromate(III), 2) chromium(III) acetylacetonate, 3) chromium(II)-hydrazinium sulfate, 4) chloride of cobalt(II) complex with thiourea, 5) potassium tetrarodancobaltate, 6) and 7) potassium bisoxalatocuprate dihydrate, 8) potassium trisoxalatoferrate(III) trihydrate, 9) potassium hexarodanonickelate hydrate, 10 and 11) manganese(III) acetylacetonate, 12) Reinecke's salt, 13) zirconium sulfide-disulfide, 14) thiocyanate derivative of cluster chromium acetate ( II) with tetraethylammonium, 15) potassium hexarodanochromate.

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

Very, very cheap Chinese achromatic plan 4×0.1 160/0.17 It turned out to be an excellent solution for the money for obtaining low-magnification micrographs. The lens copes with the task much better than conventional photographic lenses with macro rings. Another big plus is that you can purchase this lens completely new. On the downside, there is information about the existence of low-quality “evil twins” of this lens, made according to a different design: it is important to navigate by the position of the lenses in the body (along the length of the optical design) when purchasing.

You will find more reviews from readers of Radozhiva here и here.