Review of the L Plan 20x0.40 ∞/0 F=200 WD=12 planachromatic microscope objective

Material on the lens especially for Radozhiva prepared Rodion Eshmakov.

Objectives with a magnification of 20x are usually not included in the standard optics set of modern microscopes, where preference is given to a set of 4-10-40-100x. Objectives of 20x, occupying a borderline position between optics of medium and high magnifications, are purchased separately. Nevertheless, the L Plan 20/0.40 objective with an increased working distance ("metallographic") presented in the review is standard for the portable Raman microscope B&W Tek i-Raman Plus BAC151C, built according to the scheme with an "infinite" tube.

Technical specifications

Optical design – retrofocus, unknown;
Correction type – planchromat;
Tube distance – infinity, calculated for tube lens F=200 mm;
Magnification factor – 20x;
Numerical aperture – 0.4;
Focal length - 10 mm;
Working distance – 12 mm;
Cover glass thickness – 0 mm;
Compensating lens - no;
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 design of a microscope with an "infinite" tube

Microscopes with the so-called "infinite" tube have replaced old classical systems when it became necessary to create modular systems introducing new optical elements (filters, prisms, beam splitters) into the space between the objective and the eyepiece of the microscope. As is known, in a converging light beam, the introduction of a plane-parallel plate leads to a drop in image quality, while in a parallel beam it does not affect the aberrations of the system. Therefore, the overwhelming majority of modern multifunctional microscopes are built according to this scheme.

Lenses for systems with an "infinite" tube cannot be used by themselves without additional devices to obtain microphotographs without loss of quality. In fact, they are inverted and reduced copies of ordinary photographic lenses designed to work at infinity: that is, what was an image for a photographic lens is an object for a micro lens. In order for a micro lens designed for infinity to construct an image of the best quality and in the correct scale, it must be combined with another lens, which is usually called a tube lens.

"Tube lens" is (probably) a slang name for an objective that focuses a parallel beam of light coming out of a micro objective onto the camera matrix or into an eyepiece. In the simplest systems, designed for a field of no more than 22 mm, the tube lens is made as an achromatic doublet with a focal length of 180-200 mm and a relative aperture of ~1:8-1:10. In this case, when using conventional objectives with achromatic correction without varying the distance between the objective and the tube lens, the image quality level is limited by the quality of the micro objective. If, on the contrary, the micro objective has very high optical quality, a large aperture and field, and also if it is necessary to increase the distance between the tube lens and the objective (for example, to install additional modules), then the tube lens must have a more complex design. For example, some Nikon systems apparently use small anastigmatism lenses such as the Tair 200/8 (WO2023120104A1) for better correction of spherochromatism and astigmatism in the 430-650 nm range, but this is still insufficient for operation in the typical 400-700 nm range for cameras with a 36x24 mm matrix size.

The scheme of the microscope with an infinity tube can be made from ordinary photographic objectives. To do this, it is necessary to choose a long-focus objective with well-corrected chromatic aberration as a tube lens and a short-focus objective with very good correction spherochromatism as a micro lens. The long-focus lens is mounted in the straight position on the camera and focused to infinity, and the short-focus lens should be mounted in the reverse position in front of the long-focus lens. In this case, the magnification will be determined as F(long-focus lens) ÷ F(short-focus lens).
Below is an example of such a system with a magnification of 2x and a numerical aperture of 0.1 (twice that of simple 2x lenses), composed of two plan apochromatic objectives for a stereo microscope that I calculated – one 90 mm f/4.5 and the other 180/4.5.

It is important to note that there are compensation systems with an infinite tube distance, where the correction of objective aberrations (mainly lateral chromatic aberrations) can be achieved either in the eyepiece or in the tube lens. In other words, the problem of choosing lenses for photography still exists: not all lenses for microscopes with an infinite tube are capable of forming a high-quality image without an eyepiece or tube lens specially designed for them.

Lens design

The L Plan 20×0.4 lens is housed in a nickel-plated brass body. The lens is quite heavy compared to the simplest 10×0.25 achromats. The decorative part of the lens housing, as usual, is not fixed and can be accidentally twisted instead of the lens itself when trying to remove it from the microscope.

The main feature and the main advantage of the objective is a large working distance of 12 mm, allowing convenient work with thick opaque samples of arbitrary shape, as required, for example, in Raman spectroscopy. Due to the large working distance, the objective is made without using a spring-loaded fit of the lens block.

The objective lens apparently has a rather complex retrofocus optical scheme using modern highly refractive materials – otherwise it would be impossible to achieve an acceptable level of image quality at such a large working distance. According to X-ray fluorescence analysis (Bruker M1 Mistral), the front lens of the objective lens is made of heavy lanthanum flint with a refractive index of ~1.75-1.8 and an Abbe number of ~49-40.

The glass of the objective lens, when used in a Raman microscope (where it was taken from), often itself forms a background signal when recording spectra, which should be taken into account when working with low-reflecting samples.

The objective lenses have a multilayer anti-reflective coating with a green highlight color. This coating provides a much more even transmission spectrum than that used in cheap biological lenses. The short-wave transmission limit of this lens is ~370 nm, which indicates the influence of the lens materials used in the lens (heavy lanthanum and titanium-niobium flints) on absorption in the blue region of the spectrum.

Photos of the external appearance of the L Plan 20×0.4 objective, including comparison with the biological simplest 20×0.4 160/0.17 objective, are shown below.

Image quality

To conduct tests and obtain photographs, the lens was used with a new Magus Bio 250TL microscope with a trinocular attachment. The size of the image formed by this system is limited to 22 mm, so a camera was used for shooting Sony NEX-3N with an APS-C format matrix, not a full-frame one Sony A7s. The Magus Bio 250TL microscope is made using a compensation scheme, in which aberrations of the complete objectives are corrected by the eyepiece, i.e. the tube lens (achromatic doublet) in the trinocular attachment of the microscope does not participate in the compensation of aberrations.

The L Plan 20x0.4 objective has fully corrected lateral chromatic aberration by itself, and therefore forms a correct image when used with the microscope selected for testing. As befits a plan achromat, the objective has a flat field with low astigmatism and coma within a circle of 22 mm.

The main drawback of this lens is the highest level of spherochromatic aberrations. Although the spherical aberration for the green region is corrected very well in this lens, the short-wave part of the camera's working range of 400-470 nm is extremely poorly corrected in this lens, which is why the image is full of purple fringe. As a result, it turns out that in the central region of the image, the simplest biological achromat 20×0.4, which costs ~5-10 times less, builds a sharper and more contrasting image than L Plan 20×0.4, although in the field, due to uncorrected lateral chromatism, the biological lens is much worse.

The images of the reflected and transmitted light micrometers (division value 0.01 mm) on L Plan 20×0.4 are shown below.

Next is a photo taken with a biological 20×0.4 160/0.17 lens, without using a cover glass.

In my observations, the lens shows itself well in cases where the role of blue rays in image formation is minimal: yellow crystals are obtained in photos through this lens better than blue ones, for example, due to the different degree of correction of spherical aberration for different wavelengths.

It is worth noting that the lens has a shallow depth of field, so it is highly advisable to use stacking when shooting.

Below are examples of photographs taken on Sony NEX-3N through the Magus Bio 250TL trinocular using stacking.
List of objects: 1 – zirconium sulfide-disulfide, 2 – potassium oxalatocuprate, 3-4 – chloropentaamminecobalt(III) chloride, 5-6 – ammonium pentafluoroperoxotitanate, 7-8 – hexaamminenickel chloride, 9 – potassium tetrathiodanocobaltate trihydrate, 10 – bee mandibles (ready-made microscope slide).

Next, examples of photographs taken with a Sony NEX-3N through a Magus Bio 250TL trinocular without stacking.
List of objects: 1 – zirconium sulfide-disulfide, 2 – potassium oxalatocuprate, 3-4 – chloropentaamminecobalt(III) chloride, 5-7 – ammonium pentafluoroperoxotitanate, 8-10 – hexaamminenickel chloride, 11 – potassium tetrathiodanocobaltate trihydrate, 12 – bird feather (ready-made micropreparation), 13 – mosquito paw (ready-made micropreparation), 14-15 – bee mandibles (ready-made micropreparation).

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)

All reviews of microscope objectives for infinity tube:

• Review of the L Plan 20x0.40 ∞/0 F=200 WD=12 planachromatic microscope objective

Conclusions

The L Plan 20×0.4 objective has a convenient working distance and a well-corrected field, but due to the extremely poor correction of spherochromatic aberrations, it is difficult to use for taking photographs in the usual spectral range. The lens is well suited for visual devices and for photography in the limited 470-650 nm spectrum.