Lens Spotlight
Mitutoyo M Plan Apo 50x
The Mitutoyo M Plan Apo 50x was designed as a microscope objective for metallurgical applications. In this field, as well as in focus stacking, it sets the standard against which other objectives must be measured.
The Objective
The Mitutoyo M Plan Apo 50x / 0.55 NA is a high-precision microscope objective originally designed for reflective applications in industrial metal analysis. Over time, however, it has also gained an outstanding reputation among macro photographers who specialize in focus stacking. It belongs to a series of plan apochromatic objectives with various magnifications, all of which are parfocal—meaning they share the same mechanical length and working distance.
With the exception of the 1x objective, which has a larger diameter, all objectives in the series feature identical housing dimensions and thread mounts. This greatly simplifies the process of switching between magnifications in a fixed setup, as there is no need to readjust the camera extension or lighting.
These objectives are designed to be used with a standardized tube lens, typically with a 200 mm focal length, allowing for a fixed magnification at a constant working distance. The design is optimized for epi-illumination (reflected light), which not only provides excellent color rendition even under angled lighting but also enables straightforward integration into photographic setups using LED spotlights or diffusers.
The generous working distance—approximately 13 mm at 50x magnification—is a significant practical advantage. It not only helps prevent accidental contact with delicate subjects but also provides ample space for flexible and directional lighting, which is essential for creating dimensional, high-contrast images in stacked photography.
The Mitutoyo M Plan Apo 50x is part of a parfocal objective series with an identical housing diameter.
The Manufacturer
The Japanese company Mitutoyo, a global leader in precision measurement and inspection technology, is the manufacturer of this objective series. In Germany, the objectives are distributed by companies such as Edmund Optics and Novoflex.
When the M Plan Apo series was introduced, it set new standards by offering—for the first time—a combination of long working distance, high numerical aperture, excellent chromatic correction, and field-wide flatness. Depending on the tube lens and setup, the usable image circle extends to about 26 to 30 mm, making these objectives suitable for larger sensor formats such as APS-C and, under favorable conditions, even for full-frame sensors.
The optical design, however, is quite complex, resulting in an unusually large and heavy objective for microscope optics—an important consideration for mobile or lightweight systems.
Earlier lens series such as Nikon’s M Plan also delivered excellent results in metallurgical applications but were not able to combine all of the above features in the same way. Other objectives, like Nikon’s CF Plan Apo series, may achieve even higher numerical apertures and thus finer resolution, but they often involve trade-offs—such as reduced working distance or image circles that are not adequately corrected for full-frame sensors.
In addition, many of these alternatives are finite-conjugate objectives, which do not require a tube lens. Some photographers consider this an advantage, as it allows for more compact setups. However, such objectives often lack full optical correction, as classic finite objectives are usually designed to be used in combination with microscope tube lenses and/or eyepieces containing additional lenses or prisms that contribute to image correction. Without these, chromatic aberrations, contrast loss, or edge softness may occur. That said, they can still be a practical alternative for smaller sensor formats like APS-C or Micro Four Thirds.
Among experienced focus stackers, the Mitutoyo objective series is regarded as extremely reliable and high-performing. Photographers who have worked with them over extended periods often find switching to other brands to be a step backward. While these objectives are expensive, their price is justified by their precise optical correction, mechanical robustness, and consistent performance across various magnifications.
Due to their success, numerous copies have entered the market, including the HLB objectives, which are also manufactured in Japan. In addition, various no-name optics with a very similar appearance are available. These may deliver surprisingly good center sharpness in some cases, but they often show noticeable image degradation toward the edges and suffer from greater manufacturing tolerances. External similarity alone is therefore not a reliable indicator of optical equivalence. Those with the highest expectations for image quality and consistency will be well served by an original Mitutoyo objective in the long run. However, those looking to save money and willing to accept certain compromises in quality—which often go unnoticed in the final image—may find several very capable lenses in HLB’s product range.
Technical Specifications
Magnification: 50x
Numerical Aperture: 0.55
Infinity-Corrected Optics (requires tube lens)
Compatible Tube Lens Focal Length: 200 mm
Thread Diameter and Pitch: M26 x 36 TPI
Weight: 230 g
Housing Length: 66.2 mm
Housing Diameter: 30 mm
Parfocal Distance (housing length plus working distance): 95 mm
Exit Pupil Diameter: approx. 6.0 mm
Focal Length: approx. 4.0 mm
Working Distance: 13.0 mm
Resolution: 0.34 µm
Depth of Field: 0.68 µm
Manufacturer's Recommended Maximum Sensor Size: 1/2 inch
Mitutoyo specifies relatively small sensor sizes for its objectives, such as 1 inch or even just 1/2 inch—not because the optics are technically incapable of handling more, but because the company operates in industrial environments where absolute image uniformity and reproducible measurement accuracy across the entire sensor are critical. These specifications are deliberately conservative and refer to the guaranteed distortion-free image field.
In photographic practice—especially in focus stacking—we can confidently push beyond these limits. With a precisely aligned optical setup, a high-quality tube lens, and good post-processing, significantly larger sensor formats such as APS-C, Micro Four Thirds, or even full-frame can be used effectively. While sharpness and correction performance may decrease slightly toward the edges, those who know how to compensate for it benefit from a much larger image area and greater detail depth.
Imaging Performance – 200 mm Tube Lens
The following test images illustrate the imaging performance of the objective. The first is an overview shot (full-frame sensor) taken with a high-quality no-name tube lens modeled after the Thorlabs ITL 200, resulting in the nominal 50x magnification (200 mm extension). The two subsequent images each show an enlarged crop.
Test image at nominal 50x magnification on a full-frame sensor (Canon R3), using a 200 mm tube lens, with frame markers indicating the subsequent cropped enlargements — very good sharpness and no visible vignetting, largely free from chromatic aberrations (except in the extreme outer edge), and only minimal pincushion distortion, also limited to the very outer margin.
The central cropped enlargement reveals very high detail sharpness with no chromatic aberrations. There is no distortion in the central portion of the image.
In the corner crop of the full-frame image, a slight decrease in detail sharpness is noticeable in the outermost areas, particularly in the corners (here, the left side and upper left). This is because the 43 mm diagonal of the full-frame sensor significantly exceeds the image circle recommended by the manufacturer for optimal use. Mild chromatic aberrations and a more pronounced loss of sharpness can also be seen, though these effects are confined to a narrow outer edge of the image. Smaller sensors are unlikely to capture any of these imaging flaws.
Center
In the center, the objective delivers excellent resolution. The rectangular structures are clearly separated, contrast edges are sharp, and fine metal textures within the surfaces are well differentiated. Particularly noteworthy is the clean definition of the bright boundary lines—no blooming, no halo effect. Dark areas appear deep and even, indicating good control of stray light. Microcontrast is high without appearing exaggerated. In this zone, the lens reaches its full performance potential.
Extended Center
In this area, sharpness remains very high. The structures still appear cleanly contoured, with only minimal reduction in the separation between adjacent lines. The metallic surfaces continue to display a clearly visible grain, and overall image uniformity remains excellent. Contrast gradients are balanced, with no excessive highlighting of bright edges. There are no noticeable aberrations or distortion effects.
Edge Zone
Even in the edge zone, the Mitutoyo delivers impressively high image quality. The circular structures in the lower right remain mostly sharp; only under strong magnification is there a slight loss of microcontrast and a hint of softening. The rectangular edges in the upper right maintain their geometric precision. Color fringing or chromatic aberrations are not significantly present—clear evidence of the lens’s strong apochromatic correction.
Overall Impression
The Mitutoyo M Plan Apo 50x NA 0.55 provides a remarkably well-balanced, near-flawless imaging performance when used with a 200 mm tube lens. The center is sharp and high in contrast, the extended center remains fully usable, and even the edge zones maintain a convincing level of quality. This test image demonstrates that the lens delivers a broadly usable and optically stable image field—even when employed with a sensor format well beyond the specified 1/2-inch image circle—making it ideal for demanding applications such as focus stacking with high sensor coverage.
Imaging Performance – 125 mm Tube Lens
The following overview images were taken using the Raynox DCR 250 as the tube lens, which mathematically reduces the magnification to approximately 31:1. Some metallurgical microscope objectives from the aforementioned Mitutoyo series tolerate this approach, although not all to the same degree. This test is intended to show what level of image quality compromise can be expected when using the Mitutoyo M Plan Apo 50x in this configuration.
Test image with Raynox DCR 250 and 125 mm extension: This shorter tube lens focal length results in a reduced magnification, and with this combination, loss of sharpness, chromatic aberrations, and pincushion distortion can be observed in the outer edges and corners. The following cropped enlargement illustrates this more precisely. However, it’s important to note that the objective is being used here with a tube lens focal length for which it was not originally designed.
In the central cropped enlargement, detail sharpness remains quite good, although in the extended center—along the edges of the cropped area, such as on the right—noticeable color fringing is already visible.
In the corner crop of the full-frame image, a pronounced pincushion distortion is visible, which could noticeably affect the final photo. Color fringing is also more prominent here, and sharpness largely disappears; fine image details appear mushy.
Center
Despite the shortened tube lens focal length, image quality in the center remains high. The details within the square structure are cleanly separated, and the internal textures are well differentiated. Contrast edges stay defined, with no significant chromatic aberrations. However, compared to the 200 mm configuration, there is a slightly softer contrast gradient. The microscopic structures appear a bit less crisp, and there seems to be minor blooming in very bright areas. Nonetheless, performance in the center remains suitable for photographic purposes, even if not optimal.
Extended Center
In this zone, optical deviations become more apparent. The structures remain generally recognizable but lose some microcontrast. Some of the fine lines appear slightly widened, particularly along bright-dark transitions. Areas with pronounced texture or reflective elements show a degree of light scattering, likely due to internal reflections or incomplete correction of spherical aberration with this tube lens. The difference compared to the optimal 200 mm setup is clearly visible, but still tolerable when using smaller sensor formats.
Edge Zone
In the edge zone, the limitations of the optical design with a shortened optical path become especially evident. Sharpness noticeably decreases, and fine structures appear somewhat soft or blurred. The small circles in the lower area lose their defined contours, and some exhibit a halo or slight color fringing—indications of emerging chromatic aberration and stray light. Line geometry is no longer entirely accurate, suggesting slight distortion or decentering of the optical path.
Overall Impression
The image taken with the 125 mm tube lens shows that the Mitutoyo M Plan Apo 50x NA 0.55 can no longer deliver its full optical performance on a full-frame sensor when the tube distance is reduced. While the center remains usable, image quality declines noticeably outside this zone. In the outer areas, significant losses occur—in the form of softness, reduced edge clarity, and occasional reflection artifacts. For users working exclusively with smaller sensors or cropping centrally, this configuration may still be usable to a limited extent. However, to achieve maximum image quality across the full field, the objective should be operated with the specified 200 mm tube lens.
Imaging Performance – 200 mm Tube Lens: Microprocessor
A tiny microprocessor designed for mobile phone electronics features extremely delicate structures. Measuring just 3 x 3 mm in size, this image illustrates not only the sharpness and detail-resolving capabilities of a lens, but also its color rendering performance.
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 cropped image (this overview image was not taken with the Mitutoyo M Plan Apo 50x).
The Mitutoyo M Plan Apo 50x captures only a roughly 0.8-millimeter-wide section of the microprocessor shown above—delivering an outstanding rendering of minute details with excellent color contrast.
A crop from the previous image showing 0.33 millimeters of the original subject, without any post-processing—the rendering performance of this lens, which reproduces a third of a millimeter in subject width with impressive clarity, is remarkable.
The image demonstrates an overall highly uniform and optically well-balanced rendering performance. Even at first glance, the precise differentiation of the finest structures stands out: countless gold traces, contact points, and transistor networks are clearly separated, the gaps between them are deep black and free of milky stray light. Color reproduction is neutral—the gold appears metallic without artificial oversaturation, and dark textures retain their detail.
In the central areas, detail resolution remains consistently high: even the finest line grids, tightly packed conductor paths, and complex overlaps are clearly defined. Particularly impressive is the clean separation of the brightest structures without blooming—a clear indication of effective correction of spherical aberration and stray light. Color fringing is virtually nonexistent, even along highly reflective edges—an obvious advantage of the objective’s apochromatic correction.
Even as you move farther from the image center, optical behavior remains stable. Under high magnification, there is a slight softening at the extreme edges of the frame—particularly where very fine structures border directly against black—but the transition is smooth and unobtrusive. There is no abrupt drop in sharpness, only a gentle reduction in microcontrast that is barely noticeable in practice. Especially noteworthy: geometry remains accurate across the entire image field. Right angles stay square, parallel lines do not distort—evidence of very low distortion.
In the gold-toned areas, where light refracts and overlaps in multiple layers, no distracting reflection artifacts appear. Highlights remain precise, with no comet tails or diffraction rings. Darker regions of the image, such as recessed layers or vias, are clearly rendered—an indication of strong contrast performance even at low light levels.
Overall Impression
This image convincingly illustrates why the Mitutoyo M Plan Apo 50x NA 0.55 is considered one of the finest objectives in its class. Image performance is stable, high-contrast, and precise across nearly the entire field. Detail reproduction is at the highest level—even on a full-frame sensor, which takes greater advantage of the image circle than originally specified. Only in the extreme outer edges does image quality drop slightly—and even then, only in full-frame crops and under critical post-processing scrutiny.
Conclusion
The Mitutoyo M Plan Apo 50x is a high-quality microscope objective with exceptional detail resolution, outstanding color correction, and a remarkably long working distance for its magnification class. In practical use, it delivers consistently high image quality even on a full-frame sensor, whose surface area significantly exceeds the officially specified image circle of approximately 30 mm. Only at the extreme edges—and only under heavy magnification—do slight compromises become visible: minimal softening, occasional hints of color fringing, and a slight reduction in microcontrast.
These limitations, however, only become relevant when the image field is used right up to the sensor's edge. In the vast majority of real-world applications—especially with a slight crop—they are negligible. On smaller sensor formats like APS-C or Micro Four Thirds, they are virtually nonexistent. In direct comparison with the HLB Plan Apo 50x, the Mitutoyo clearly demonstrates more balanced imaging behavior, particularly at the edges. The usable image circle is larger, the transition to the periphery is more precisely controlled, and overall image uniformity is superior.
The situation changes when the objective is used with an overly short tube lens—such as 125 mm instead of the intended 200 mm. In this case, the optical design reaches its limits: while the image center remains usable, the edge zones show a significant drop in image quality. Beyond extensive sharpness loss, light reflections, stray light, and instability in bright structures appear—clear signs of an improperly compensated optical path. On full-frame sensors, this configuration is only recommended to a limited extent from an optical standpoint. For smaller sensors or centrally cropped subjects, it may be acceptable; for wide-field imaging, however, it is not.
All in all, the Mitutoyo M Plan Apo 50x remains an outstanding objective, well suited for photographic use on full-frame sensors—provided one understands the limits of the usable image circle and takes into account the optical deviations caused by altered tube lengths. Its strengths lie in its extraordinary sharpness, excellent color and contrast correction, high manufacturing precision, and parfocality across the entire objective series—a major advantage in setups involving variable magnifications. For those willing to accept or deliberately compensate for the slight quality drop at the extreme edge, the Mitutoyo M Plan Apo 50x offers a tool of impressive optical precision—even well beyond its originally specified application scope.
Advantages
Extremely long working distance, high image sharpness and detail accuracy, excellent color correction, and parfocality within the objective series, allowing for easy lens changes.
Disadvantages
High purchase price; slight edge softness on full-frame sensors when using a shorter tube lens (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.
Fazit