Lens Spotlight
HLB Plan Apo 50x
The HLB Planapo 50x 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 50x was developed for metallurgical applications and belongs to a series of parfocal objectives with varying magnification levels. All models in the series share identical threads, very similar housing diameters, and the same parfocal distance—that is, the consistent total length from housing to working distance.
This parfocality is particularly advantageous when used in specialized microscopes where both the camera mount and often the tube lens are fixed. However, this design also proves beneficial in focus stacking setups: swapping objectives within the series requires no further adjustment—the setup remains unchanged.
The optics of the HLB Plan Apo 50x are designed for reflected light, which supports accurate color reproduction, especially on highly reflective surfaces—a key advantage over many traditional microscope objectives that are optimized for transmitted light applications.
A long working distance is often essential in metallurgical contexts, as light must frequently be directed in from the side. In this respect, the HLB objective differs significantly from typical laboratory objectives used in medicine or biology, where transmitted light is standard and a long working distance is neither required nor desirable.
For focus stacking in particular, the extended working distance is a major advantage. In combination with the appropriate tube lens, the objective can be easily adapted to a full-frame camera, and the generous space between lens and subject greatly facilitates flexible lighting.
The HLB Plan Apo 50x is part of a parfocal objective series with a uniform housing diameter.
The Manufacturer
The HLB Plan Apo 50x originates from the Japanese company Shibuya Optical, which develops objectives, specialized microscopes, and other optical instruments for a wide range of professional fields. In Europe, the objective series is distributed by Stonemaster (www.stonemaster.de).
The technical inspiration likely came from the high-end objective series by Mitutoyo, which set industry standards with its parfocality, long working distance, and excellent imaging performance—but at a significantly higher price point. The HLB series positions itself as a cost-effective alternative and now offers image quality that is, in many respects, comparable to the Mitutoyo line.
Additionally, there are numerous Chinese knockoffs that closely mimic the external design of the Mitutoyo objectives and are sold at even lower prices. However, this visual similarity should not be mistaken for equivalent optical performance—especially at high magnifications, where quality differences become immediately apparent.
Technical Specifications
Magnification: 50x
Numerical Aperture: 0.55
Infinity-corrected optics (requires a tube lens)
Compatible tube lens focal length: 200 mm
Thread diameter and pitch: M26 x 0.706
Weight: 325 g
Housing length: 82 mm
Housing diameter: 35 mm
Parfocal distance (housing length plus working distance): 95 mm
Exit pupil diameter: 5 mm
Focal length: 4.0 mm
Working distance: 13.0 mm
Resolution: 0.5 µm
Depth of field: 0.9 µm
Imaging Performance – 208 mm Tube Lens
The following test images illustrate the optical performance of the objective. The first shows an overview capture (full-frame sensor) using the Raynox DCR 150 tube lens with a focal length of 208 mm, which results in approximately the nominal magnification of 50x. The two subsequent images each show an enlarged crop.
Test image at nominal magnification (DCR 150), with frame markers for the subsequent cropped enlargements — sharp rendering of even the most delicate details, though sharpness noticeably decreases as the eye moves from the central area toward the edges. However, the color correction is good, as the bluish tint on some of the tiny surfaces is due to the natural coloration of the subject being imaged.
The central cropped enlargement shows good detail sharpness. Here too, the bluish tints are not chromatic aberrations but rather a characteristic of the subject being imaged.
The cropped enlargement from the edge and corner area reveals the broad extent of sharpness loss. Additionally, there is a noticeable pincushion distortion (visible here toward the upper right) in the full-frame image.
The image shows exceptionally high detail clarity, especially in the central region. Even the finest structures—such as rectangular conductor geometries, edge boundaries, and metallic grids—are rendered with crisp precision. Microcontrast in the image center is pronounced, with sharp transitions between bright and dark areas. This reinforces the impression that the HLB occasionally appears even slightly sharper and more contrast-rich than the Mitutoyo M Plan Apo 50x in the center—particularly when resolving extremely fine structures.
Light Behavior and Reflections
In highly reflective areas (e.g., within the metallic fields in the upper right), however, initial differences become apparent: Compared directly with the Mitutoyo, the HLB shows a somewhat greater tendency toward light blooming under strong local contrast. Bright structures tend to “bloom” slightly, and fine textures may wash out on highly reflective surfaces. This might suggest slightly less effective stray light suppression or a marginally weaker control of spherical aberration—though still at a very high optical standard.
Edge Zone
In the peripheral areas of the image—especially in the lower right and upper right corners—a noticeable drop in sharpness begins to emerge. Fine lines lose some separation clarity, the contrast between adjacent structures becomes flatter, and the precision of boundary lines decreases. These effects are not dramatic, but they are noticeable—particularly in large-format viewing or when compared directly with an image taken using the Mitutoyo M Plan Apo 50x, which exhibits a more gradual falloff in sharpness.
Color and Contrast Reproduction
Color rendering appears neutral to slightly cool, with a subtle violet-gray tint in the metal layers—likely caused by lighting conditions or camera white balance rather than the lens itself. Chromatic correction is strong; there are no visible color fringes or aberrations, which clearly indicates apochromatic design quality.
Overall Impression
With a 208 mm tube lens and full-frame sensor, the HLB Plan Apo 50x delivers an impressively high level of detail resolution in the image center, with strong microcontrast and sharp line definition. Toward the edges, moderate sharpness and contrast falloff become evident under close scrutiny, making the HLB slightly more limited than the Mitutoyo when the entire image field is used. Overall, the HLB produces a very high-quality visual result, with strong, contrast-rich central rendering, albeit with less uniformity across the full image field.
Imaging Performance – 125 mm Tube Lens
The following overview images were taken using the Raynox DCR 250 tube lens, which mathematically reduces the magnification to approximately 31:1. Some metallurgical microscope objectives from the aforementioned HLB series tolerate this configuration to varying degrees, and the purpose of this test is to determine what level of image quality trade-offs can be expected when using the HLB Plan Apo 50x under these conditions.
Test image with DCR 250: The reduced tube lens focal length results in a lower magnification. With this setup, the pincushion distortion in the outer edges and corners becomes even more pronounced. The loss of sharpness also increases significantly toward the corners.
The central cropped enlargement still shows reasonably good detail sharpness, but it’s questionable whether there’s a significant advantage over the 20x lens from the same series—even though the NA value is higher at 0.55 (compared to 0.42 for the 20x).
The cropped enlargement from the edge and corner areas no longer shows any sharp details; here, the image circle of the objective is clearly being pushed far beyond its limits.
Center
In the center of the image, the level of detail remains decent—line structures and metallic conductor paths are still cleanly separated, and microcontrast is acceptable. However, when compared directly with the 200 mm or 208 mm configuration, there is a noticeable loss of sharpness even in the central area: edges appear slightly softer, fine textures look somewhat smudged, and the overall impression is less crisp and precise. The objective’s full imaging potential is clearly not being utilized here.
Extended Center
Just outside the central zone, image quality begins to degrade significantly. Structures appear increasingly blurred, lines lose definition, and the clarity of fine detail drops off rapidly. This decline in sharpness extends across a broad area and affects not just the extreme corners, but also mid-frame regions—typical of suboptimal optical correction due to incorrect tube lens distance. Contrast also drops noticeably, giving the image a somewhat “muddied” appearance.
Edge Zone
In the outer edge areas, image quality is clearly compromised. Lines blend together, and the overall impression is soft and slightly imprecise. Structures look almost as if seen through a subtle soft-focus filter. This suggests that at 125 mm tube length, the optical path no longer lies within the correction range of the objective.
Lighting and Flare Behavior
A marked increase in flare and slight halos around very bright structures is also noticeable. This indicates increased stray light, possibly caused by internal misfocusing or slight field curvature that can no longer be sufficiently compensated for at the shortened tube distance. Color fringes or chromatic aberration, on the other hand, are barely perceptible—the color correction remains stable.
Overall Impression
Using the HLB Plan Apo 50x with a 125 mm tube lens (e.g., Raynox DCR-250) on a full-frame sensor is not recommended when the entire image field is to be used. Even in the image center, full resolution is not achieved, and in edge zones, quality drops off sharply. For smaller sensor formats or compositions centered tightly around the optical axis, this setup may still be marginally acceptable—but even then, the visual result clearly falls short of the objective’s potential.
Using the correct 200 mm tube lens (or close to it, such as 208 mm) is strongly recommended for this lens in order to fully realize its optical capabilities.
Comparison HLB Planapo 50x – HLB Planapo 20x
The direct comparison with the Mitutoyo HLB Plan Apo 20x shows that the HLB Plan Apo 50x (both with tube lens 208 mm) delivers slightly more detail. However, this gain is nowhere near as significant as the price difference between the two objectives—especially considering that the comparison somewhat underrepresents the 20x, since its image was upscaled much more than that of the 50x.
Left: HLB Plan Apo 50x, right: HLB Plan Apo 20x – the 50x reveals a hint more detail than the 20x, but the difference is surprisingly small.
Imaging Performance – 208 mm Tube Lens: Microprocessor
Extremely fine structures can be seen in a tiny microprocessor designed for smartphone electronics. Measuring just 3 x 3 mm, the image of this chip demonstrates not only the sharpness and detail resolution of a lens, but also its color rendering capabilities.
The microprocessor shown at its full width of 3 mm, slightly cropped at the top and bottom. The frame indicates the area shown in the following detail image (this overview image was not taken with the HLB Planapo 50x).
The HLB Planapo 50x captures only a 0.8-millimeter-wide section of the microprocessor shown above—delivering an excellent rendering of the tiniest details with very good color contrast.
A cropped section from the previous image, showing 0.33 millimeters of the original subject, entirely unedited—while the advantage of this lens over the 20x from the same series may appear surprisingly small, its imaging performance is simply impressive. Reproducing a third of a millimeter of subject width with stunning clarity—without any image processing!
Central Zone
In the center of the image, the HLB Plan Apo 50x delivers very good resolution. Fine features like logarithmically nested conductor paths, complex circuitry patterns, and grid-like rectangular areas are sharply defined and rendered with depth. Microcontrast is high, and even darker metallic layers reveal clear details. In this zone, the performance comes close to that of a Mitutoyo—sometimes even offering slightly crisper contrast.
Extended Center and Edge Zones
However, moving away from the center, the HLB 50x begins to show its known weakness: a noticeable drop in sharpness toward the image periphery. Fine lines become less precise, contrast slightly fades, and certain areas develop a faintly milky appearance. The upper right corner—near the dense switching structures—makes this effect particularly evident: micromotifs there lose definition that is clearly visible in the center.
Light Behavior and Color Neutrality
Despite the LED illumination, color reproduction remains pleasantly neutral, with a slight golden tint caused by the structure itself. There are no obvious color fringes or halo effects at edges. Reflective surfaces are rendered calmly and without distracting flicker. Stray light and internal reflections are well controlled.
This image demonstrates that, with optimal tube lens focal length (200 or 208 mm), the HLB Plan Apo 50x can deliver excellent results even on a full-frame sensor—particularly in the image center. The visual impression is sharp, contrast-rich, and highly detailed. However, the edge zones show more pronounced sharpness falloff than the Mitutoyo—something to consider when choosing a sensor or planning for cropping. For centrally placed subjects with high crop potential, the HLB remains a powerful and cost-effective alternative.
Overall Impression
At first glance, the image conveys a very uniform and contrast-rich impression. The processor’s structures are consistently well differentiated—even in densely packed areas with high conductor line density. The optical rendering is clear, calm, and rich in detail, with no distracting highlights or noticeable noise.
Resolution Test
The Zeiss Resolution Test 300 allows the resolution of a microscope objective to be read as a numerical value. While the interpretation of this reading is somewhat subjective and not entirely precise (see details here), it still provides a useful general indication of fine detail rendering and image sharpness.
The resolution value, visible here in the two outer fields, was measured at the center of the lens as 1600 line pairs per millimeter (lp/mm).
Conclusion
The HLB Plan Apo 50x, when used with a 208 mm tube lens on a full-frame sensor, delivers an impressively high level of detail resolution in the center, with strong microcontrast and crisp line definition. However, in the peripheral areas, a noticeable drop in image quality becomes apparent: sharpness decreases, and contrast softens—significantly more so than with the Mitutoyo. The overall image quality remains high but is less uniform across the entire field.
Compared to the 20x objective from the same series, detail reproduction is slightly superior—which is understandable given the only moderate increase in numerical aperture from 0.42 to 0.55. Those familiar with more dramatic jumps, such as from 5x to 10x (NA 0.14 to 0.28), should expect a more subtle improvement here.
Once you move away from the central zone, image quality declines quickly. At nominal magnification (with a 200 mm tube lens), the unsharp edge area remains relatively narrow—sufficient for unimpeded use with APS-C or MFT sensors. However, with shorter tube focal lengths like 125 mm (e.g., using a Raynox DCR-250), edge sharpness drops significantly on a full-frame sensor.
This may give the impression that the quality difference compared to the 20x lens is minor and could be compensated by upscaling. But that’s misleading: images from the 50x objective provide significantly more latitude for cropping and enlargement—reserve that the 20x lens has already maxed out. Whether this gain justifies the considerably higher investment is up to each user to decide.
This objective is especially recommended for smaller sensors. On full-frame, it delivers convincing results only if edge cropping is planned or the subject is clearly centered. Use with a 125 mm tube lens, however, is hardly advisable—this configuration clearly reveals the optical limitations.
Advantages
Extremely long working distance, high image sharpness and detail accuracy in the central region, good color correction, parfocal design within the objective series, allowing for easy lens changes.
Disadvantages
On full-frame sensors with a 200 mm tube lens, slight softness appears at the outer image edges; with a shorter tube lens (125 mm), this softness extends across a broader peripheral zone.
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.
Fazit