Lens Spotlight
HLB Plan Apo 20x
The HLB Plan Apo 20x is a microscope objective designed for metallurgical applications. This test aims to evaluate how well it performs in focus stacking setups without the use of a dedicated research microscope.
The Objective
The HLB Plan Apo 20x was developed for metallurgical applications and belongs to a parfocal objective series with varying magnification levels but uniform physical specifications: all models share the same housing length, working distance, diameter, and thread size.
This parfocality not only simplifies use on specialized microscopes with fixed camera mounts and integrated tube lenses, but also proves advantageous for focus stacking: switching between objectives in the series requires no changes to the setup.
Since the 20x objective is optimized for reflected light, it often delivers better color rendition than traditional transmitted-light objectives typically used in medical or biological applications.
Another major benefit is the long working distance, which is essential for oblique or side lighting—common in metallurgical imaging. Unlike traditional lab objectives with shorter distances, this feature significantly facilitates illumination.
This strength becomes particularly evident in a focus-stacking setup using a full-frame camera: the lighting setup is straightforward, and the system remains flexible—even for high-resolution captures of complex technical structures.
The HLB Plan Apo 20x is part of a parfocal objective series with a consistent housing diameter.
The Manufacturer
The optics are manufactured by the Japanese company Shibuya Optical, which produces a wide range of specialized microscopes, objectives, and optical accessories for various professional fields. Distribution in Europe is handled by Stonemaster (www.stonemaster-onlineshop.de).
The design was likely inspired by the well-known parfocal objective series from Mitutoyo, which set industry standards with its long working distance and outstanding image quality—albeit at a very high price point. HLB positions itself as a competitive alternative offering comparable performance at significantly lower cost.
Numerous Mitutoyo clones from China closely resemble the original in appearance but typically fall short in optical performance, despite their attractive pricing.
Technical Specifications
Magnification: 20x
Numerical Aperture: 0.42
Infinity-corrected optics (requires tube lens)
Compatible tube lens focal length: 200 mm
Thread diameter and pitch: M26 x 0.706
Weight: 270 g
Housing length: 75 mm
Housing diameter: 35 mm
Parfocal distance (housing length plus working distance): 95 mm
Exit pupil diameter: 10.4 mm
Focal length: 10.0 mm
Working distance: 20.0 mm
Resolution: 0.65 µm
Depth of field: 1.60 µm
Imaging Performance – 208 mm Tube Lens
The following test images illustrate the optical performance of the lens. The first shows an overview shot (full-frame sensor) taken with the Raynox DCR 150 tube lens, producing approximately the nominal magnification of 20x. The two subsequent images each present an enlarged crop from the overview.
Test image at nominal magnification (DCR 150), with frame markers indicating the subsequent cropped enlargements – clear detail reproduction with high sharpness, no edge vignetting, but slight pincushion distortion in the peripheral and corner areas.
The central cropped enlargement shows good detail sharpness. The slight blooming is due to the shooting method without a diffuser.
Cropped enlargement of the top-left corner: In the outer edge area, especially in the corners, a decrease in detail sharpness is noticeable, as the 43 mm diagonal of the full-frame sensor exceeds the image circle of the objective. Some slight chromatic aberrations are also visible here. However, smaller sensors are unlikely to capture these imaging defects.
The test image taken with the HLB Plan Apo 20x (NA 0.42) on a full-frame sensor reveals an overall impressive optical performance, especially given the high magnification and the demanding sensor size.
Center
In the image center, the objective delivers excellent detail resolution. Even the finest structures within the complex microcircuitry are clearly separated. Microcontrast is strong, with no visible chromatic aberrations or optical artifacts. Black lines appear rich and sharp, free from fringing or blurring. Tonal gradations in the grayscale range are well differentiated, indicating precise tonal rendering and high optical accuracy.
Extended Center
In the extended center—regions about one-third to halfway from the image center to the edge—image quality remains outstanding. Contrast is slightly reduced compared to the central zone, but still very well defined. Line rendering stays crisp, and even complex structures remain clearly delineated. There are no signs of field curvature or spherical aberration, suggesting that the flat-field correction of the objective is performing as intended.
Edge Zone
In the outer image areas, sharpness decreases moderately but noticeably. Microcontrast drops somewhat, and extremely fine structures begin to blur slightly. Still, performance remains impressive: there are no significant chromatic errors, no evident field curvature, and no astigmatic distortions. The rendering remains visually coherent, though no longer at the flawless level seen in the center. For a 20x industrial-grade lens used on a full-frame sensor, this is still considered a remarkably strong result.
Overall Impression
The HLB Plan Apo 20x demonstrates very high imaging performance in the test image. Resolution is excellent in the center and extended center, with only moderate softening at the far edges—well within acceptable limits. The precision of line rendering, resistance to optical artifacts, and excellent field flatness qualify this lens for demanding applications in technical macro photography and microstructure imaging—especially when the sensor area is used primarily in the center, as is often the case in focus stacking applications.
Imaging Performance – 125 mm Tube Lens
The following test image is another overview shot, but this time captured using the Raynox DCR 250 tube lens, which reduces the magnification to approximately 12.5x (instead of 20x). Some metallurgical microscope objectives from the mentioned HLB series tolerate this configuration to a certain extent, although not all respond equally well. The purpose of this test is to evaluate what level of image quality compromise can be expected with the Mitutoyo Plan Apo 20x under such conditions.
Test image with DCR 250: The shorter tube lens focal length results in a lower magnification, and with this combination, noticeable pincushion distortion and a clear decrease in detail accuracy appear toward the edges and corners. A hint of chromatic aberration is also visible, as some of the fine elements appear more colorful here than in the center. However, these CAs are not yet strongly pronounced.
The central cropped enlargement is still free of CAs and shows good detail sharpness.
Cropped enlargement of the upper left corner – here, distortion and blur become significantly more pronounced toward the edge of the frame. The color correction also appears to be overwhelmed; subtle color shifts are visible along the narrow horizontal structures. Additionally, the entire corner appears to have a yellowish tint.
The test image demonstrates the optical performance when deviating significantly from the intended system focal length of 200 mm. This configuration not only reduces the effective magnification but also alters the beam’s angular aperture, which directly impacts image quality.
Center
In the center of the image, the resolution remains at an acceptable level. The line structures are still discernible, but microcontrast is lacking—fine details appear softer compared to when the correct tube lens is used. Structures are visually separable, yet they appear somewhat flatter and less three-dimensional.
Extended Center
In the extended center area, image sharpness continues to decline. Contrast diminishes, and in tightly spaced circuit patterns, the delineation between lines begins to blur. Particularly in small, densely packed features, it becomes clear that resolution is no longer sufficient to fully separate the image information.
Edge Zone
In the edge regions, image degradation becomes more severe. The loss of sharpness is unmistakable: lines become ragged, details appear smudged, and microcontrast drops noticeably. Slight vignetting or reduced brightness may also be caused by the altered beam geometry. Field curvature and other aberrations are more prominent than when the lens is used within its intended optical design.
Overall Impression
This test image clearly shows how sensitive a high-quality industrial objective like the HLB Plan Apo 20x is to optical mismatches. Image quality suffers even with moderate deviation from the recommended tube lens focal length—especially in combination with a full-frame sensor and complex visual structures. To take full advantage of this lens’s capabilities, a 200 mm tube lens should absolutely be used. Only then can its high numerical aperture be translated into precise resolution and strong contrast.
Imaging Performance – 208 mm Tube Lens: Microprocessor
A tiny microprocessor designed for mobile phone electronics features extremely delicate structures. Measuring just 3 x 3 mm, its image reveals not only the resolving power and detail reproduction of a lens but also its color rendering capabilities.
Excellent sharpness, even in the finest details; color contrast is somewhat flat. Minor distortions are present in the corner areas, though they are barely noticeable visually, since the overview image was not cropped and thus includes the full edge zones of the full-frame sensor.
These two test images provide a clear demonstration of the imaging performance of the HLB Plan Apo 20x under optimal operating conditions with a tube lens focal length of approximately 200 mm. The lens was used with a full-frame camera, and the subject is a 3 × 3 mm smartphone microprocessor. The first image shows the full field of view at 20× magnification, while the second presents a highly magnified detail from the previously marked region.
The overview immediately reveals how finely structured and complex the microprocessor layout is at this level of magnification. Despite the immense information density, the image remains clear and stable—an early indicator of the lens’s high optical quality. The uniform illumination and the absence of significant edge artifacts suggest that the lens performs well across the entire sensor area, even on a full-frame sensor.
The detailed view powerfully reinforces this impression: even densely packed structures with minimal contrast differences are cleanly separated. Line rendering is precise, transitions appear sharp and well-controlled, and even the finest elements remain distinguishable without blurring or overlapping. Microcontrast is high but not artificially exaggerated—especially evident in finely meshed areas that often pose problems in many test setups.
The lens also demonstrates strong resistance to chromatic aberration. Color fringes at contrast edges are virtually nonexistent—an impressive achievement at this magnification level and with reflected light illumination.
Color reproduction in the detailed image is also balanced and nuanced. Different material zones on the processor can be clearly distinguished without color casting or artificial-looking oversaturation. This is a major advantage of metallurgically optimized reflected-light objectives over traditional transmitted-light microscope lenses, which often struggle with such contrast conditions.
Overall, the HLB Plan Apo 20x delivers consistently impressive image quality in this test. Sharpness, contrast, color fidelity, and structural separation are all at a very high level—especially considering the sensor format and actual image size. The test images clearly demonstrate that this lens is highly suitable for demanding documentation tasks, microstructural analysis, and scientific macrophotography.
Resolution Test
The Zeiss Resolution Test 300 makes it possible to determine the resolving power of a microscope objective in the form of a numerical value. While this reading is somewhat subjective and not entirely exact (see here for details), it does provide a general impression of the objective’s fine detail rendering and resolution performance.
The resolution value, shown here in the two outer fields, was measured at the center of the lens as 1250 line pairs per millimeter (lp/mm).
Conclusion
The HLB Plan Apo 20x impresses overall with very high optical quality that can rival significantly more expensive top-tier objectives in many areas. When used with the correct tube lens focal length (200 mm, slightly exceeded here at 208 mm), the objective delivers excellent central resolution, with clean separation even of densely packed structures. Particularly in critical areas such as memory arrays or finely structured logic segments, the line definition remains sharp and high in contrast. The numerical aperture of 0.42 allows for very fine differentiation—a point where the high level of correction becomes clearly evident.
Image quality also remains consistently high across the extended central field. Only at the outermost edges is there a slight drop in contrast, but without noticeable distortion, field curvature, or chromatic aberrations. The overall rendering appears sharp, dimensional, and color-neutral throughout—ideal for documentary and analytical use in technical macro photography.
In practical use—such as focus stacking with full-frame sensors—the HLB Plan Apo 20x fully plays to its strengths: Its high optical performance is complemented by a comfortable working distance, a consistently flat plane of focus, and a robust mechanical design that enables seamless switching between different magnification levels within the HLB series.
For users seeking top-tier imaging performance in the 20x class while maintaining an optimal price-performance balance, the HLB Plan Apo 20x offers a highly capable and field-proven alternative to established high-end objectives on the market. Its performance—especially when used with the ideal tube lens focal length—is consistently strong and well suited even for the most demanding tasks.
Advantages
Extremely long working distance, high image sharpness and detail accuracy, parfocal design within the objective series allowing for easy lens changes, and a price advantage compared to the Mitutoyo reference objective.
Disadvantages
Unusual thread size that requires a special adapter (e.g., www.stonemaster-onlineshop.de); certain quality losses when using a shorter tube lens focal length (DCR 250).
Daniel Knop, www.knop.de, www.danielknop.eu
Testbild mit DCR 250: Im Zentrum ist die Bildschärfe bei dieser Kombination nur moderat und deutlich geringer als bei der Nominalvergrößerung, und außerhalb des Bildzentrums lässt sie gewaltig nach. Hier zeigt sich auch eine leichte kissenförmige Verzerrung. Die Abdunklung des Rand- und Eckenbereichs ist deutlicher als bei Verwendung der DCR 150.
Das Bildzentrum hat noch gewisse Schärfe, aber feinste Details werden in Kombination mit der DCR 250 nicht mehr wiedergegeben.
Die Randzone weist starke kissenförmige Verzerrung und intolerable Unschärfe auf, die zur Ecke hin extrem wird (hier links oben). Im Vollformat ist diese Kombination aus Objektiv und Tubuslinse schlicht unbrauchbar.
Der direkte Vergleich mit dem Canon-Lupenobjektiv MP-E 65 mm bei Stellung 3,5x zeigt, dass das HLB Planapo 3,5x diesem sehr scharf abbildenden Makrospezialisten deutlich unterlegen ist. Das Canon bringt mehr Schärfe (Bild oben rechts), und der Schärfeabfall zum Bildrand und vor allem zu den Ecken hin ist beim Canon deutlich schwächer als beim HLB. Allerdings muss hier auch berücksichtigt werden, dass das HLB Planapo 3,5x neu weniger als die Hälfte dessen kostet, was für ein Canon MP-E 65 mm zu veranschlagen ist.
Vergleich HLB M Plan 3,5x – Canon MP-E 65 mm
HLB Planapo 3,5x (links) im Vergleich mit dem Canon MP-E 65 mm bei Stellung 3,5 (rechts), oben jeweils das rechte obere Viertel des Originalbilds, aufgenommen mit Vollformatsensor (Focus Stack), unten jeweils ein Sechzehntel des Originalbilds, entsprechend hochskaliert.
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