Venus Laowa 25mm f/2.8 2.5-5X Ultra Macro,
tested at 4x

Very few current produced photomacrography lenses exceed a 2x magnification unaided (i.e., without using some kind of bellows, extension tubes, tube lens or add-on front lens). Among these, only three reach 5x:

• Canon MP-E 65 mm f/2.8 1-5x Macro Photo
• Zhongyi Mitakon Creator 85mm f/2.8 1-5X Super Macro
• Venus Laowa 25 mm f/2.8 2.5-5x Ultra Macro

All three lenses cover a full-frame sensor, and they share the same fundamental design, i.e. :

• Manual operation of the "focus ring". In practical use, the focus ring is rarely used for focusing, and has instead the function of changing magnification. Autofocus would be largely useless in these lenses, since at these magnifications DOF is so shallow, that most of the time autofocus is likely to choose the wrong part of the subject to focus onto.
• Using these lenses requires a mechanism (usually a focusing rack or a micrometric linear rail) to finely adjust the distance between the subject and the camera + lens assembly. With the possibly single exception of a legacy Minolta photomacrography lens, this mechanism is not a part of the lens.
• Lens aperture is fully manual, except in the Canon lens. Automatic or preset aperture can be advantageous with the optical viewfinder of a DSLR, in order to provide as bright as possible a picture in the viewfinder. The Canon MP-E 65 and its automatic aperture were designed to facilitate framing and focusing in the context of this optical viewfinder. With live view now optional in virtually all current DSLRs, and obligatory in mirrorless cameras, an aperture that closes to the chosen value just before the exposure is no longer necessary for a comfortable framing and focusing of the subject, especially if artificial illumination is available. I prefer a fully manual aperture that shows me the actual DOF of the image, so that I can made the best use of it.

The Canon MP-E 65 has been around for quite several years, and its design has never been updated or improved. Canon is still offering it, in spite of past rumors that it had been discontinued. A problem with this lens is that it lacks a physical aperture ring. Adapting this lens for use on non-Canon cameras requires the use of a "smart" adapter capable of translating the electronic signals produced by the camera body into signals that the lens can understand, or tortuous shenanigans involving setting the lens aperture on a Canon camera, then moving the lens to the camera used to record images without the lens resetting its aperture. Another problem with this lens is that the front diameter of the lens barrel is so large compared with the diameter of the front optical element, and the working distance of the lens so short, that at high magnification the front of the barrel makes it difficult to suitably position the light sources.

The two remaining lenses are more recent offerings, and are available in a variety of full-frame lens mounts. While the Zhongyi lens has a built-in tripod collar, the Laowa lens does not come with one, and Laowa sells it separately as an option. This tripod collar, however, is insufficiently stiff, and I prefer to use the tripod shoe of a Metabones Nikon F to Sony FE adapter, further equipped with a long Arca-compatibe plate that adds a convenient way to coarsely adjust the distance to the subject (Figure 1, rightmost).

The Zhongyi 85 mm f/2.8 is a good idea crippled by a faulty design and a poorly thought-out fix of the fault. In 2019, this lens caught the attention of several photographers specializing in photomacrography when it became known that Zhongyi had sent test prototypes of a 1x-5x lens to a few professional photographers. Some of the latter, within the limitations of their non-disclosure agreements, expressed very high opinions on the performance of these prototypes. Once the first production batch of the lens reached buyers in late 2019, however, it quickly turned out that lens performance was quite poor in terms of both resolution and contrast. Clearly, Zhongyi had done some changes to the production batch that ruined the lens performance - one of the many examples of cost-cutting, corner-cutting practices common in Chinese companies and known as "quality fade". Typically, quality fade is a slow process that gradually takes place during the months or years a given item is being manufactured. In this case, quality fade seems to have been instantaneous, and to have taken place between the prototype manufacturing and the first production batch.

A quick fix of the problem with the original Zhongyi 85 mm f/2.8 came out in early 2020, and involved a much smaller front optical element and some internal changes. These changes did seem to partly solve the observed problems, but also reduced lens speed by about two stops, thus preventing the lens from producing its intended image resolution, especially above 2x. Making things even worse, Zhongyi never acknowledged this new problem. Zhongyi did not even change the pictures of the lens on their web site, which still (as of April 2022) show the initial, faulty version no longer available for sale.

Incidentally, this was not the first time Zhongyi had failed to produce a lens capable of providing its intended performance. The Mitakon Zhongyi 20mm f/2 4.5x Super Macro launched a few years earlier had apparently been designed to provide an extended magnification range, but prototype tests showed it was usable only within the very restricted magnification range of 4.0x to 4.5x. Rather than pulling the lens from production and heading back to the design board, Zhongyi decided to re-tool the production line to limit the travel of the focus/magnification ring, revise the nominal specifications, and go ahead with the sales anyway. This seems to indicate a toxic corporate culture where designs known to be faulty are not stopped from entering production. Instead, the worst of the faults are just plastered over rather than their root causes being corrected, and the still unsatisfactory products pushed onto unsuspecting customers.

Thankfully, Venus Laowa can design and build far better photomacrography lenses than Zhongyi, and is now known for products of high quality at reasonable prices. After the innovative but not fully satisfactory 60 mm f/2.8 2x, we have seen a quickly diversifying range of Laowa photomacrography lenses of better and better performance, as well as a few extreme lens designs previously unavailable in consumer products.

Laowa 25mm f/2.8 2.5-5X

In spite of providing only half the magnification range offered by its Canon and Zhongyi competitors, the Laowa 25 mm is remarkable for its good image quality, sufficiently fast lens speed, relatively narrow diameter of the front lens barrel, reasonable working distance even at the highest magnification, and solid construction. At 416 g, and considering this is a relatively small lens, it feels hefty and well-built. The magnification ring turns evenly and without wobbles, and the length of the lens in Nikon F mount increases from 82 to 137 mm thanks to a system of three snugly fitting coaxial barrels (including the fixed one carrying the lens mount). The front of the lens does not rotate when changing magnification.

The optical scheme uses 8 elements (one of which made from a low-dispersion glass) in 6 groups, and is therefore moderately complex by current standards. At least one floating group seems to be present at the rear of the diaphragm. All elements are thicker than usual, in spite of their small diameters. The 8-blade diaphragm sits behind the two first optical elements and is close to the front of the lens. In comparison, the Canon MP-E 65 uses 10 elements (of which one low-dispersion) in 8 groups and a floating subassembly, and the Zhongyi 85 mm uses an undisclosed optical formula with a floating subassembly.

The optical scheme is obviously asymmetric, and the pupil ratio (defined as P_rear / P_front ) computes to 1.86 at 2.5x and 1.79 at 5x. The rear pupil is very recessed within the barrel, especially at 5x, but I succeeded in imaging it with the Laowa 100 mm f/2.8 Apo at 1x.

A rectangular baffle flush with the lens mount is meant to eliminate stray light caused by unused portions of the image circle. This baffle moves within the barrel independently of the optical groups when magnification is changed, presumably in order to remain optimally placed at each magnification.

Some users reported an uneven vignetting in one or more corners when this lens was mounted on a T2 or M42 adapter. This was apparently caused by these screw mounts not perfectly aligning the rectangular baffle with the sensor. Although one user reportedly solved this problem by removing the baffle, I would suggest instead to properly align the baffle to the sensor by loosening the adapter's set screws and rotating the threaded mount of the adapter. This is possible in T2 adapters and most M42 adapters, but not in some T adapters.

The front element is mounted flush with the convex front of the barrel, and there is no front filter mount. If a filter is necessary, it should be mounted at the rear of the rectangular baffle.

A proprietary bayonet mount surrounds the front of the barrel. It is used to mount a proprietary front lens cap of metal-clad plastic. A small ring light, available at extra cost, can be mounted here. A tripod collar is also available. As mentioned above, I do not regard it as sufficiently stiff, and prefer to use an Arca-compatible plate integral to the Nikon F to Sony EF Metabones adapter.

The focus/magnification ring has a travel of approximately 100° and causes two coaxial sleeves to extend forward by about 55 mm. One of these sleeves is engraved with a magnification scale with markings at 2.5, 3, 3.5, 4, 4.5 and 5x that makes it easy to set the desired magnification. The aperture ring is located at the front of the barrel and has full-stop clicks between 2.8 and 16.

It is useful to remember that E (effective aperture) is given by:

E = N (M + 1)

where N is nominal aperture (read from the aperture scale), and M magnification. This is an approximate formula that does not take P (the pupil ratio) into consideration. Taking the nominal aperture at face value and for the moment ignoring P, the effective aperture (rounded to the closest integer) at the magnifications available on this lens is:

Laowa provides essentially the same data of effective aperture in the user instructions of the lens, albeit only at nominal f/2.8 (rounded to the first decimal, which makes no practical difference).

On the other hand, if we introduce P in the above formula, we obtain:

E = N (M / P + 1)

This is a more correct formula that gives significantly different results when P is substantially different from unity, like in this case. Applying this formula gives:

A comparison of the two tables shows that the effective aperture values at the same nominal aperture and magnification are quite different, depending on how they are computed.

The question is now whether Laowa's lens designers adjusted the values of the nominal aperture of the lens in the specifications of the lens, in order to take P into account. This would be an unorthodox way of specifying the nominal lens speed, but could make some kind of sense in a lens that does not focus at infinity. The alternative is that Laowa's technical writers simply applied the approximate thin-lens formula when writing the lens documentation, without caring about P and its effects. For the moment, I will not attempt to directly answer this question.

The diameter of the front element of the lens, in a lens that only focuses at magnifications substantially higher than 1x, is not a reliable indicator of lens speed. The 15 mm diameter of the front element, plugged into the formula N = D / F (where N is nominal aperture, D the diameter of the front element and F the focal length) produces f/1.6, far from the nominal f/2.8 specification. This formula does tell us that the nominal lens speed cannot be faster than f/1.6, but it can be slower.

A high diameter of the front element can be a result of multiple design choices, including the need to avoid vignetting and darkened corners at all available magnifications. A front pupil placed in the air in front of the lens is technically possible, and is frequently another factor that requires a larger front element. This, however, does not apply to the present lens, where the front pupil is just behind the front element. Thus, answering the above question is not trivial. In the following discussion, I conservatively use the effective apertures as specified by Laowa, as a worst-case specification.

Working distance is 45 mm at 2.5x and 40 mm at 5x. The relatively narrow (40 mm diameter) front of the barrel helps to illuminate the subject. However, an even narrower front diameter should be possible, and would further help. The main limitation is probably the diameter of the diaphragm housing, which is close to the front element, while the aperture ring could be moved a bit farther to the rear.

The following table shows the approximate field of view, in mm, recorded by this lens on a sensor of a given size (FF = 24x36 mm, APS-C = 18.7x24.9 mm, MFT = 13.5x18 mm). Be aware that there is no "standard" APS-C size, and APS-C sensors can somewhat vary in size among and within camera brands. Although easily computed, this table can be practical to have when setting an initial magnifications to use for a given task.

Tests

Robert O'Toole tested this lens at 2.5x and 5x (nominal magnifications), and reported that the actual maximum magnification is 4.82x. In order not to unnecessarily duplicate his test, I am presenting detailed results only at nominal 4x.

The subject of this test (Figure 3) is the tip of the tail of the same subject used for tests of the Laowa 100 mm f/2-8 CA-Dreamer Macro 2x. The cropped detail (Figure 4, 5, 6) is from mid-left to the image center. Image quality at nominal f/2.8 (effective f/14, Figure 4) is good but (as expected) slightly less excellent than with the best lenses at lower magnifications, with finest detail not much larger than one pixel in the areas in best focus. Some of the other areas, however, are blurred because they are out of focus, as a consequence of the low DOF. At nominal f/5.6 (effective f/28, Figure 5), the image is slightly blurred by diffraction, but DOF is significantly higher. Depending on the use of this image and the nature of the subject, an aperture in the interval between f/2.8 and f/5.6 could be the best compromise. At nominal f/11 (effective f/56, Figure 6) the image is significantly blurred and, unless the pixel count is substantially reduced, image quality is too poor in spite of the further increase in DOF.

If a low-resolution image for web publication is all that is needed, nominal f/8 and even f/11 might still be good enough. If the best image quality is required, f/2.8 should be used, probably combined with focus stacking if the subject is even slightly three-dimensional.

Figure 4 shows some axial chromatic aberration (green out-of-focus blobs, especially at the top and at the left of the subject), that becomes much less evident after closing the aperture to nominal f/5.6. This is a larger amount of this type of chromatic aberration than I have seen in other tests of the same lens. The color and relief of this particular subject may be partly responsible for this.

The Laowa 25 mm 2.5-5x in focus stacking

Figures 7-9 are examples of focus stacks at 2.5x and 5x, respectively, on a 42 Mpixel full-frame sensor. The images were recorded on a Sony Alpha 7 II under continuous illumination by two small LED panels placed very close to the subject, and were fused in Zerene Stacker 1.04, a popular program used for this task since 2009 and capable of handling large stacks of hundreds of pictures. The default settings of Zerene Stacker were used (to make comparisons easier, although these settings may not be optimal for all subjects), with just a single manual adjustment performed during the stack processing. No slabbing was used. The individual JPG images of the stack were loaded into Zerene Stacker unedited, straight out of the camera.

At 5x, each pixel corresponds to 0.9 μm on the subject. The nominal f/2.8 lens speed, together with the measured pupil ratio of 1.86 at 2.5x and 1.79 at 5x (see above), allows a diffraction-limited CoC (circle of confusion) diameter of approximately 1.5 pixels at 2.5x and 3 pixels at 5x (i.e., at 5x, details roughly 3-4 μm across). The camera itself is capable of resolving detail of less than 2 pixels in optimal conditions, so the lens is the limiting factor at 5x, while lens and sensor are fairly well matched at 2.5x (considering the lens, simplistically, as diffraction-limited and "perfect" in all other respects).

This lens turns out to be a practical choice for focus stacking. It is not on par with top-of-the-line fixed-magnification objectives like the Mitutoyo M Plan Apo series, but stacks shot at nominal f/2.8 within the nominal magnification range of this lens are quite detailed, even on a 42 Mpixel sensor. The amount of real detail at 5x is clearly higher than at 2.5x, with some, but not much, empty magnification. In particular, the 5x image makes the appearance of the central cap in each setal socket clearer than at 2.5x, and in some places one can even see part of a ring of very fine setae surrounding the socket of large setae, right at the resolution limit of the system. These fine setae are not resolved in the 2.5x image. The semi-transparent, shiny stray fiber with an apparently purple core is also much clearer in the 5x image.

The evaluation of focus stacks is more difficult, and to some extent more subjective, than the evaluation of a single, unprocessed image. The software used to generate the fused image from the individual images composing the stack typically uses two or more alternative algorithms, and each algorithm can be tweaked by changing the values of a number of parameters. Therefore, this software can potentially affect in multiple ways the final appearance of the fused image.

In addition, virtually all stacking algorithms have known weaknesses, most often in the rendering of overlapped features so distant from each other along the depth axis that when one of them is in focus, the other is invisible in the background or foreground blur. Different algorithms may produce different artifacts in these conditions, and techniques are available to give better results in particular situations like this one. Therefore, numerous reservations must be kept in mind when evaluating a focus-stacked image. Nonetheless, it may be useful to see here an example of largely unedited stacks shot with this lens and fused with one of the most popular stacking software.

I processed the same image stack with Helicon Focus 7.6 (not the latest version, but nonetheless the one I have access to) with default settings, and its three stacking methods gave results a little different from Zerene Stacker. With the present stack, I would give the results of Zerene Stacker a slightly better rating, although this may vary with other subjects.

Helicon Focus seems to be developed mainly for relatively large subjects, while Zerene Stacker seems to be most often used in extreme macro and microscopy. Also, Helicon Focus can do a substantial part of its repetitive number-crunching in the GPU, if your computer is equipped with one of the supported, modern graphic cards. From a development point of view, this means that the source code of Helicon Focus, or at least some of its libraries, may need to be updated often to allow an optimal use of the latest GPUs. Zerene Stacker seems to do all its number crunching in the CPU, which is much slower but does not need frequent code updates. If you need to process dozens of large focus stacks on a daily basis, e.g. on a PC in the photography lab of a research institution or museum, the different speed of the two programs may make a significant difference.

Different types of licenses are available for both programs. Fully paid (i.e. non-expiring) licenses of Helicon Focus currently seem to be somewhat more expensive than the corresponding licenses for Zerene Stacker. However, do your own shopping when deciding the purchase of specific stacking software and licenses.

Color, contrast and magnification range

With this lens, color and contrast are good at all magnifications. Color rendering is consistent with the Laowa 100 mm f/2.8 2x., so these two lenses can be used as a set covering the magnification range between zero (i.e. infinity focus) and 5x without significant interruptions. Probably, most of the other Laowa models of 2x macro lenses, especially the ones specified as Apo, are also suitable to complement the Laowa 25 mm f/2.8 2.5-5x (with the likely exception of the original Laowa 60 mm f/2.8 2x).

The necessity of swapping lenses at one point in this magnification range, and the physically very different sizes and working distances of these lenses, make bridging across the "break" in magnification range time-consuming, especially in the field. This is partly compensated by the fact that the Laowa 25 mm, coupled with one of the Laowa 2x lenses, provides a better image quality of both currently available lenses that offer a magnification range between 1x and 5x. In addition, these lenses provide a magnification range that does require lens swapping at 1x, instead of 2-2.5x.

A few specialized lenses, some of them individually even better than the Laowa lenses discussed herein, combined together can provide a reasonably full coverage of the same magnification range. However, each of these lenses typically excels only in a restricted range of magnifications, requiring frequent lens-swapping when changing magnification, and sometimes also color and contrast adjustments in post-processing to better match each other.

Conclusions

The Laowa 25 mm f/2.8 Ultra Macro is a versatile lens providing a good image quality already fully open, in a magnification range from 2.5x to (almost) 5x and at a reasonable price. This magnification range complements the several models of Laowa macro lenses capable of magnification up to 2x, and the magnification gap between 2x and 2.5x is not a significant drawback in practical use of these lenses.