Lens Spotlight
Mitutoyo M Plan Apo 7,5x
The Mitutoyo M Plan Apo 7.5x was designed as a microscope objective for metallurgical applications. In this field—as well as in focus stacking—it sets the standard by which other objectives must be measured.
The Objective
The Mitutoyo M Plan Apo 7.5x is part of a parfocal objective series developed for metallurgical applications. All objectives in this series share the same housing diameter (with the exception of the 1x model), the same thread, and identical working distances. This allows for seamless interchangeability—a benefit that proves valuable not only in specialized reflected-light microscopes with fixed camera extensions and tube lenses, but also in focus stacking setups.
The objective is optimized for reflected light and therefore provides color rendering that is often limited in transmitted-light objectives. The large working distance—typical of Mitutoyo—makes top or side lighting much easier, a decisive advantage over many conventional laboratory objectives used in medicine or biology, where compact design and short working distances were sufficient or even preferred.
Although designed for use in industrial microscopes, the Mitutoyo M Plan Apo 7.5x is ideally suited for photographic focus stacking. When combined with a tube lens and a full-frame camera, it allows for easy and flexible illumination of the subject—especially with three-dimensional, technically detailed objects.
The Mitutoyo M Plan Apo 7.5x is part of a parfocal objective series with an identical housing diameter.
The Manufacturer
The manufacturer of these specialized objectives is the Japanese company Mitutoyo, renowned worldwide for its high-precision measurement and inspection systems in the field of metallurgy. In Germany, these objectives are available from Edmund Optics (www.edmundoptics.de) and Novoflex (www.novoflex.de), among others.
When first introduced, the Mitutoyo series set new standards: it combined a long working distance, high numerical aperture (NA), excellent detail reproduction, outstanding color correction, and virtually distortion-free imaging all the way to the edge of an unusually large 30 mm image circle. Additionally, all objectives in the series are parfocal—a significant advantage in practical use. This combination was made possible by an exceptionally large lens diameter, which brought size and weight to levels previously rarely seen.
While excellent metallurgical objectives already existed—such as Nikon’s “M Plan” series—the Mitutoyo line introduced a new standard in the totality of its optical performance. Other optics, like Nikon’s “CF Planapo,” may offer even higher resolution due to their greater NA, but this often comes at a cost, such as reduced working distance or a smaller image circle. The latter can lead to edge softness or color fringing when used on full-frame sensors.
One advantage of classic finite-conjugate objectives is that they don’t require a tube lens, which remains attractive for many users—especially those working with smaller sensor formats. Ultimately, choosing the Mitutoyo objective series is a matter of preference and conviction. However, those who work extensively with focus stacking and high-resolution macro photography often stay with Mitutoyo optics—thanks to their consistent performance and robustness.
Due to the high price of the originals, a number of replicas have entered the market. These include the HLB objectives, which are also manufactured in Japan and are covered on this site as well. There are also various no-name copies that closely resemble the original in appearance and are sold at much lower prices. However, one should not assume that this visual similarity translates to comparable optical performance.
This objective was kindly provided for testing by Dr. Kristian Peters.
Technical Specifications
Magnification: 7.5x
Numerical Aperture: 0.21
Infinity-corrected optics (requires tube lens)
Compatible Tube Lens Focal Length: 200 mm
Thread Diameter and Pitch: M26 x 36 TPI
Weight: 240 g
Housing Length: 60.0 mm
Housing Diameter: 34 mm
Parfocal Distance (Housing Length + Working Distance): 95 mm
Exit Pupil Diameter: 13 mm
Focal Length: 26.67 mm
Working Distance: 35.0 mm
Resolution: 1.3 µm
Depth of Field: 6.2 µm
Imaging Performance – 208 mm Tube Lens
The following test images illustrate the imaging performance of the objective. The first shows an overview shot (full-frame sensor) taken with the Raynox DCR-150 tube lens, resulting in an approximate nominal magnification of 7.5x. The two subsequent images each show a magnified crop.
Test image at nominal magnification (DCR 150), with frame markers for the following cropped enlargements – very good sharpness, no visible vignetting, free of chromatic aberrations, and very slight but barely noticeable pincushion distortion toward the image corners.
The central cropped close-up shows, as expected, very good detail sharpness without chromatic aberrations.
In the enlarged corner crop of the full-frame image, a decrease in detail sharpness is visible in the outer edge area (here in the upper left), and at most, there is a faint hint of chromatic aberration. However, this is limited to a narrow outer margin of the image; on a smaller sensor (APS-C, MFT), these effects would almost certainly not be visible.
Center
The image captured with the Mitutoyo M Plan Apo 7.5x demonstrates overall very high imaging quality, particularly in the center. Even the finest structures are rendered sharply and with strong contrast. Line definition remains crisp, even in very small details, and there are no signs of color fringing or chromatic aberration. Color reproduction appears clean and neutral, even in complex grayscale transitions.
Extended Center
This high level of quality is largely maintained throughout the extended central zone. Sharpness decreases only minimally—barely enough to matter in typical applications. The image remains distortion-free and geometrically precise, thanks to the lens's well-engineered optical correction. No chromatic effects are detectable in this area either.
Edge Zone
In the outer image zone, a slight drop in sharpness becomes apparent upon close inspection. The finest structures appear slightly softer, and very thin lines lose a bit of edge clarity. While the image circle is sufficiently large for full-frame sensors, the optical performance approaches its limits in this region. However, field curvature remains low, there's no abrupt falloff in sharpness, and vignetting continues to be negligible. Color correction also remains stable out to the edges, which speaks strongly to the optical quality of this lens series.
Overall Impression
Overall, the Mitutoyo M Plan Apo 7.5x delivers excellent performance on a full-frame sensor, more than sufficient for most focus stacking applications. The lens offers consistently high image quality with outstanding color correction, exceptional flatness of field, and a generous working distance that simplifies subject illumination. Even in the outer areas, image quality remains at a high level, despite the slight softening typical of microscope objectives when used with large sensors. For anyone working with high precision in technical or natural science fields, this objective is an excellent choice—powerful, durable, and optically superb.
Imaging Performance – 125 mm Tube Lens
The following test image is another overview shot, this time taken with the Raynox DCR-250 tube lens—well below the lens's specified focal length of 200 mm. As a result, the effective magnification is approximately 4.69:1 instead of 7.5:1. This has a noticeable impact on optical performance, as the test image clearly demonstrates.
Test image with DCR 250: The shorter tube lens focal length results in a lower magnification, and with this combination, vignetting is already visible around the edges—particularly in the corners. Sharpness also drops off significantly in those areas.
In the central cropped close-up of the full-frame image, detail sharpness is consistent and good. No chromatic aberrations are visible, and the image is free of distortion.
In the enlarged corner crop of the full-frame image, both distortion and blurring increase noticeably toward the edge.
Center
In the center, image quality remains high but is no longer flawless. The finest details are generally recognizable, but even here the image appears somewhat duller and flatter compared to optimal tube lens configuration. Sharpness is slightly reduced, and microcontrast is weakened—fine structural differences begin to blend in places, and lines appear slightly blurred. Grayscale gradations are also less nuanced than under ideal conditions. This is typical when using an infinity-corrected objective with too short a tube lens, as the light path no longer runs through the intended correction zone.
Extended Center
This impression continues in the extended central zone. Details remain visible, but slight softness becomes more noticeable, which cannot be attributed to focusing errors but rather to a combination of field curvature and resolution loss. Very thin line structures lose edge clarity, and the distinct separation of individual circuit paths starts to fade. Overall image fidelity is still good but clearly limited. Contrast ratios are also reduced, especially noticeable in subtle grayscale transitions.
Edge Zone
In the edge zone, quality degradation becomes clearly visible. Noticeable softening occurs—beyond what would typically be seen with this lens on a full-frame sensor in an optimal setup. Lines begin to fray, and microscopic details appear blurred or slightly distorted. This is likely due in part to field curvature and aberrations that are no longer adequately corrected when the tube lens deviates from specification. Minor chromatic errors or color fringes may also occur—not dramatic, but visible upon close inspection. Brightness remains largely consistent, with little to no vignetting.
Overall Impression
In summary, when used with a 125 mm tube lens, the Mitutoyo M Plan Apo 7.5x still delivers usable image quality but clearly falls short of its true potential. Sharpness is reduced across all image zones, contrast is noticeably lower, and full-field correction is no longer achieved. While the lens still produces acceptable results in the center, edge performance is significantly compromised for high-resolution applications. For demanding focus stacking on full-frame sensors, a tube lens close to 200 mm should be used to access the lens’s full optical capability. In this shortened configuration, the lens remains functional, but it loses precision, uniformity, and brilliance—qualities that are critical in microscopic photography.
Imaging Performance – 208 mm Tube Lens: Microprocessor
A tiny microprocessor designed for mobile phone electronics features extremely delicate structures. Measuring just 3 × 3 mm, it provides an image that reveals not only the sharpness and detail resolution of a lens, but also its color rendering capabilities.
The approximately 1.5 mm wide crop of the 3 × 3 mm microprocessor shows clear, sharp details. Color contrast is somewhat flat.
Center
In the center of the image, the optical performance is outstanding. Fine structures of the silicon chip—including densely packed transistor rows and wiring layers—are rendered with exceptional sharpness and detail fidelity. Transitions between light and dark areas are razor-sharp, with no halos or bleed. Microcontrast is also excellent—even subtle textures and brightness variations within larger functional blocks remain clearly distinguishable. Color reproduction is neutral, with no visible chromatic aberrations or color fringing.
Extended Center
In the extended central zone, this quality is nearly maintained. Line rendering remains precise, contrast stays high, and even the finest substructures are reproduced cleanly. There are no noticeable distortions or color errors here either. Particularly impressive is that even where structural lines run diagonally or horizontally across the chip, no astigmatic distortion or shape deviations are visible—clear evidence of the lens’s excellent field correction.
Edge Zone
In the peripheral areas, the Mitutoyo M Plan Apo 7.5x continues to perform very well. Fine conductor traces, gate patterns, and vias remain sharply resolved. While a minimal loss of sharpness can be detected compared to the center—especially in extremely intricate patterns—the image quality still far exceeds what many comparable objectives can deliver at the edges of a full-frame sensor. There is no noticeable vignetting, color fringing, or distortion. Field curvature is so minor that even at a flat focal plane, no significant softening occurs.
Overall Impression
Overall, this test image compellingly demonstrates the optical class of the Mitutoyo M Plan Apo 7.5x in its intended configuration. Image quality is nearly uniform across the entire frame and allows for high-precision technical reproduction on full-frame sensors, with excellent color and structural fidelity. This makes the lens ideally suited for detail-critical applications such as photographic documentation of microelectronic components, where absolute precision and minimal imaging errors are essential. The combination of high numerical aperture, perfect color correction, and generous image circle makes this lens a reference tool for high-magnification focus stacking applications.
Resolution Test
The Zeiss Resolution Test 300 allows the resolution of a microscope objective to be assessed in the form of a numerical value. While this reading is somewhat open to interpretation, not entirely exact, and also highly dependent on the resolution of the camera sensor (see details here), it still provides a rough indication of fine detail reproduction and rendering precision. With the objective discussed here, the resolution test remains somewhat problematic—even with the use of a high-resolution 45-megapixel sensor—because the smallest resolvable line spacings are so fine that they are rendered by only a few pixels, making them harder to distinguish. In general, objectives with a magnification of 7.5x or lower actually require smaller sensor pixels for more accurate readings. Starting at around 10x, readings become more reliable.
The value for the resolvable line pairs was measured here at 560 lp/mm. However, when using a camera sensor with smaller pixels, it might be possible to achieve a slightly higher reading.
Conclusion
The Mitutoyo M Plan Apo 7.5x is a high-quality microscope objective that excels in focus stacking applications—not only for technical subjects, but also for biological and artisanal motifs. As with all lenses in this series, its optical performance impresses with excellent detail reproduction, outstanding color correction, and uniform illumination across the image field. Compared to an upscaled photo taken with the 5x HR objective of the same series—with an identical numerical aperture of 0.21—the advantage of lossless optical magnification over digital enlargement is clearly evident: the 7.5x delivers noticeably more fine detail and clarity.
When combined with a full-frame sensor, the lens does approach its physical limits—especially at the image periphery and corners—yet even there, overall image quality remains at a good level. In centrally composed subjects or classic “fly-head photography,” where the focus lies on the image center anyway, the slight edge softness is practically irrelevant and poses no issue for the vast majority of typical applications.
For those requiring uncompromising sharpness right to the corners—such as with flat, structured objects on full frame—the HR version is an option, though it comes at a significantly higher price. A more pronounced quality drop occurs when the lens is used with a too-short tube lens, such as 125 mm instead of the specified 200 mm. In such cases, the optical corrections shift enough to cause a noticeable loss of sharpness and microcontrast in all areas of the image. This setup is not recommended.
It’s important to keep in mind that this objective was originally designed for sensor formats up to a maximum of 2/3 inch. On that smaller image area—where only the central region of the optics is used—performance is uncompromisingly high. Formats like Micro Four Thirds or APS-C come closer to the lens’s critical image circle edge but still show no significant optical defects. Even on full frame, the drop in sharpness at the extreme corners is minor enough to be negligible in most practical scenarios. Whether this small drawback is acceptable ultimately depends on the user's expectations and application.
In sum, the Mitutoyo M Plan Apo 7.5x is an excellent objective offering high resolution, strong field flatness, reliable color fidelity, and an ideal working distance. When used correctly—with an appropriate tube lens and suitable sensor size—it delivers reference-level performance for high-resolution macro photography.
Advantages
Extremely long working distance, high image sharpness and detail accuracy, excellent color correction, and parfocality within the lens series—making lens changes quick and easy.
Disadvantages
High purchase price, uncommon thread size requiring a special adapter (e.g., www.stonemaster-onlineshop.de), slight edge softness when using a shorter tube lens focal length (e.g., DCR 250).
Daniel Knop, www.knop.de, www.danielknop.eu