Photomacrographic lenses, part 4:
Industrial macro zooms:
Unitron Zoom 1:6.5, Navitar Zoom 6000, Optem Zoom 70  

Figure 1. Top: Optem Zoom 70 with aperture control, fine focus module, 2x additional lens and non-inverting 90°prism.
Middle: Unitron Zoom 1:6.5 with RMS to C-mount adapter tube.
Bottom: Navitar Zoom 6000 with fine focus module.

On an earlier page, I reviewed the Unitron Zoom 1:6.5 (Figure 1, middle), an industrial photomacrographic zoom lens which provides a parfocal 1:6.5 zoom ratio and a magnification range between 0.7x and 4.7x when mounted 170 mm from the sensor plane. The numerical aperture ranges from 0.026 (at the low end of the magnification range) to 0.070 at the high end. The mount uses a standard RMS thread. A sleeve threaded on both internal (RMS) and external (C-mount thread) surfaces is also shown. It is used to mount the lens at the bottom of a Unitron tube with an internal diameter of 23 mm (i.e., for standard microscope oculars). This sleeve provides approximately 20 mm of fine focusing at the expense of varying the tube length, thus causing the lens not to be perfectly parfocal and requiring refocusing when the zoom factor is changed. Alternatively, the fine adjustment of the tube length can be used to achieve perfect parfocality by trial and error, and locked afterwards.

The Navitar Zoom 6000 (Figure 1, bottom) is another lens with the same optical specifications as the above. As far as I can see, the optical elements and internal mechanical assemblies of these Unitron and Navitar lenses are identical down to the smallest detail (with the exception noted below). I am convinced that the base optical assemblies for both lenses come from the very same factory and production line. Externally, these lenses look quite different from each other. The Zoom 6000 uses a rather wide (33.3 mm at its largest point) conical bayonet to attach to a microscope, and has a small zoom ring accessible through two slots of the barrel. The Navitar Zoom 6000 is either a re-branded Optem RetroZoom 65 (or vice versa), or a lens using the same optical subassembly in a slightly different barrel.

My specimen has a 3mm fine-focusing range controlled by the rubber-coated ring near the bottom of the lens. This ring moves the front element group back and forth. It has a very small range of rotation (less than 45°) and is difficult to control with precision. Some of the other models have a 10 mm fine-focusing range. The zoom and fine-focusing rings can be locked by using small thumb screws that are provided with the lens. If used, the zoom thumb screw is unavoidably going to visibly mar the metal surface of the ring.

Figure 2. Optem Zoom 70, with variable aperture option.

The Optem Zoom 70 (Figure 1, top, and Figure 2) was introduced in 2001 and, according to its manufacturers/marketers, is a modern design, while the two other lenses discussed above are said to use a 30-year-old design. While this statement seems to imply that the Zoom 70 is a better design than its predecessors, I would take it with a grain of salt. After all, the Zeiss Luminars and other photomacrographic lenses are just as old, but still unrivalled. The Zoom 70 provides a slightly longer zoom range (0.75x to 5.2x, or a 1:7 zoom ratio). In spite of this, it has a wider numerical aperture (0.024-0.080), which potentially means a higher maximum resolution as limited by diffraction, and the front lenses of the lower module are much wider than those of the zooms described above.

The Zoom 70 is composed of two modules. The upper module contains the zoom optics and, if available, the variable aperture. The lower module contains a front optical group and, depending on the chosen option(s), a fine-focus mechanism moving this group up and down, a front threaded mount for attaching add-on lenses, and/or axial illumination optics. Alternatively, the front module may contain an adapter for mounting special objectives or infinity-corrected microscope objectives in place of the front optical group. Although the lower module can be detached from the upper one, the upper (zoom) module cannot be used alone, without any of the lower modules.

The zoom scale ranges from 1 to 7, but the upper module has a built-in magnification factor of 0.75x. This is not the result of an internal reduction lens, but simply a design specification that cannot be changed. Just take this reduction factor into account when you compute the final magnification. The lens attaches to a microscope by a 1" x 32T thread (i.e., mechanically the same as a C-mount commonly used for video lenses, but of course not with the same register distance). In its original configuration (Figure 1, top), my specimen connected to the rest of the system through a 1/2" long adapter that converts the C thread to a 25.2 mm conical bayonet, used by the other accessories of the system. This bayonet is similar in construction, but much smaller than the one used by the Navitar lens discussed above. There is an even smaller (23.7 mm) bayonet used in items branded Navitar, so you must make sure which one you actually need when ordering the parts for building a system.

This lens is shown in the above pictures, and normally is used by myself, with a matched 2x add-on lens screwed into the front mount. This increases the numerical aperture (and therefore, potentially increases also the maximum resolution), decreases the working distance to roughly half, and increases the zoom range to 1.5x-10.4x. Using this add-on lens places the Zoom 70 wholly in the photomacrographic range, where it is most useful to me (I find it more confortable to use conventional macro lenses at 1x and below).

Note that there is also an Optem Zoom 70XL, which is externally identical but manufactured to provide an extended mechanical life without servicing. I believe that this difference is relevant only to motorized zooms used in an industrial setting, the zoom ring of which is operated very frequently by servo motors and wears out much more quickly than under manual operation. For photomacrography, and especially if you look for a second-hand item, try to get a manual (either XL or non-XL) model instead. The Zoom 70XL is currently in production, but I cannot find the Zoom 70 among current offerings. There is also a Navitar Zoom 7000, which is a completely different lens from those discussed on this page.

The knurled ring at the top of the zoom module is a built-in option of this module (i.e., an option that must be specified when ordering the zoom module, and cannot be added later). It is available for virtually all these zoom models, albeit it is rarely seen in material advertised on eBay. It is a variable aperture ring that controls an internal diaphragm (Figure 2). This is very desirable in general photomacrography, because it allows a control over DOF (depth-of-field), compatibly with diffraction. However, this feature is rarely seen in industrial applications of these lenses. I believe that the main reason for this is that this type of lens is typically used for inspecting semiconductor wafers and PC boards. Therefore, the subject is very flat, and needs a very limited DOF. These lenses provide their best resolution with the aperture fully open, and closing down the diaphragm reduces resolution. Nonetheless, there are subjects and situations in which a higher DOF may be worth a loss in fine detail. The diaphragm of the Zoom 70 has a large number of blades (at least 17), and is quite well rounded in outline. It closes from 11 mm down to 1.2 mm, or roughly 7 stops. Closing it down excessively does cause a visible loss of resolution caused by diffraction.

Disassembling and realigning these lenses

Figure 3. Disassembling the Navitar Zoom 6000.

The Navitar Zoom 6000 can be disassembled by unscrewing three set screws near the rear of the lens mount (Figure 3). The screw heads may be factory-filled with epoxy, which can be removed with a metal dental pick or a coarse needle with a dull point (in order not to scratch the metal parts). These screws can also be used to align the markings on the zoom ring with the index mark on the fixed barrel, which sometimes are misaligned in second-hand specimens. Realigning requires the screws to be only slightly unscrewed, just enough to turn the barrel within the sleeve. The Optem Zoom 70 can be disassembled and realigned in the same way. In lenses that have a variable aperture, like the Zoom 70 discussed above, the aperture ring may need to be removed first (by unmounting the lens from the microscope/camera tube, which frees the aperture ring) to access the set screws.

The front group of the Unitron Zoom 1:6.5 can be unscrewed by hand, but further disassembly requires a special spanner wrench. The Unitron lens can be realigned  by loosening two small set screws on the barrel near the mount.

In all these lenses, the barrel should be aligned so that the zoom ring stops turning when the lowest magnification mark is aligned with the index. At the opposite end of the zoom range, the Unitron and Navitar lenses slightly exceed the marking of the highest magnification value. This is normal, because the last marking is 4.5x, while these lenses reach 4.7x. Note that Optem and Navitar zooms have two windows that expose the zoom ring, but only one of these carries an index mark. The two windows are necessary to grab the zoom ring with opposing fingers. This ring cannot be operated reliably with just one finger.

I recommend that you do not disassemble the zoom barrels into their components, and especially that you do not remove optical elements from the barrels, unless absolutely necessary (i.e., to remove fogging or large foreign particles that do affect image quality). Removing lenses from their mounts may require re-alignment with instruments available only in the factory that assembled the zoom barrels, in order not to degrade the optical performance of the system.

Choosing a zoom model and accessories

Additional models of macro zoom lenses for industrial use, with zoom ratios up to 16, are also available from the above companies, and additional ones. I have not tested any of them. Judging from their specifications they are at least as good, in terms of resolution and low distortion, as the models discussed on this page, provided that they are used on relatively small sensors (1/2 to 3/4 in.). They do not seem to be adequate for the larger sensors of DSLRs, unless additional optics are used. This can be solved by using a 2x optical group mounted in the camera tube, albeit at the price of a matching reduction in absolute resolution at the camera end. This optical group must be of good quality and designed specifically for use with a large sensor, lest it produces unacceptable aberrations. Regardless of this, the use of some of the zooms with the highest zoom ratios (especially 1:10 and above) with large sensors is not recommended by their manufacturers, even with a 2x adapter. On the other hand, zooms with a more limited zoom ratio, like those discussed on this page, perform much better with large sensors and often produce an acceptable coverage of a DSLR sensor without needing such an adapter. Lenses that use camera tubes with a large internal diameter (e.g., Navitar ones) are less likely to cause vignetting than those that attach to a standard 23 mm microscope tube.

Edmund Optics used to sell re-branded Navitar models (albeit not currently the 6000). The similarities beween some of the models of different brands are discussed above. Therefore, it seems that one of these companies, or possibly yet another company (my guess: Optem/QiOptiq), acts as a provider of preassembled optical systems and subsystems to be marketed under different brands. In practice, this means that by shopping around and being aware of the different brands you might be able to get the same optics at a lower price.

Costs for new specimens of these lenses are relatively high (roughly between 600 and 1500 €, depending on base module and accessories), but not much more than a good macro lens for DSLR cameras. Second-hand and even unused, still sealed specimens are frequently sold on eBay, sometimes for as little as 1/3-1/4 of their original prices. Accessories are harder to come by on eBay. Often they are bundled with more or less complete second-hand systems, and isolated accessories sometimes are advertised at unreasonably high prices, in some cases exceeding even the list prices for new equipment. As a whole, these industrial zoom lenses are quite within the reach of serious amateurs. However, knowledge of these lenses seems to be practically absent among enthusiasts of photomacrography. Virtually all discussions of photomacrographic lenses I have seen in advanced amateur forums concentrate on discontinued specialist lenses like the Zeiss Luminars and Leitz Photars, or on the use of enlarger lenses and other optical systems not primarily designed for this purpose.

These zoom lenses are primarily designed for use with high-resolution videocameras (although naturally they can be used also in microscopes for direct visual observation, as well as coupled to still cameras) and are available in a number of basic configurations. Some models have the zoom ring, and sometimes a fine-focus ring, coupled to electrical motors. These models are intended for computer-controlled operation, often in clean-room environments, are more expensive, and are not desirable for manual use in photomacrography. If necessary, the motors and gears can be removed, and the lens controlled manually. However, if offered a second-hand motorized zoom, avoid specimens that have been worn mechanically by prolonged use (which is the main cause of failure for these lenses). The models best suited for photomacrography, and also cheaper, are instead those with manual zoom control. Some of these models also have an additional ring (usually near the front of the lens) for fine focusing. This feature can be useful, but is not indispensable. In any case, a focusing rack must be added because the range of built-in fine focus is insufficient for general use. The Unitron zoom adds fine focusing in the form of a threaded sleeve that connects to a microscope tube of standard diameter (Figure 1).

Since the variable aperture is mounted at the back of the optical elements (unlike in general-purpose camera lenses, in which typically it is located between optical elements), some ingenuity may allow the addition of this feature in lenses that lack it. For instance, instead of a diaphragm, one could add a slider carrying three or four drilled holes of different diameters as fixed apertures, thus providing essentially the same function in a much simpler way. For semi-permanent use, a narrower aperture can be implemented as simply as by placing a black-painted washer of the right size somewhere near the lens mount. In these lenses, the optional diaphragm is mounted roughly 10 mm at the back of the rear lens surface, and quite close to the lens mount. Its distance from the rear element does not seem to be very critical, and probably an aperture can be placed within several mm from this distance without causing any vignetting.

Other options and additional modules include coaxial illuminators, screw-on matched lenses to increase or reduce the total magnification, polarizers, analyzers and DIC prisms, adapters for mounting infinity-corrected microscope objectives in front of the zoom and boost considerably the total magnification (above 1000x in some cases), tubes for connecting the lens to different types of video and still cameras, and optical adapters to cover sensors of different sizes (albeit, see my comments at the bottom of this page).

An additional factor to consider is that the 24 by 18 mm sensor of a typical Nikon DSLR is equivalent to a 30 mm image circle, i.e., substantially more than 1 inch. Several industrial zooms, especially those with high zoom ratios, do not provide image circles sufficiently large, and cause a substantial vignetting in the corners (and/or other aberrations near the corners) with these cameras.

Wild/Leica Macrozoom and Apozoom

Wild, and later Leica, made (and Leica still makes) high-end "photomacroscopes", i.e., compound microscopes designed for photography in the photomacrographic range, primarily between roughly 8x and 40x. They use a high-quality parfocal zoom lens (either the 1:5 Macrozoom of 1:6 Apozoom) comparable to, albeit much larger, heavier and expensive than, the ones described in this page. These lenses, however, are designed for use with a tube lens built into Wild/Leica macroscopes. I further discuss these zooms and their microscopes here.

Image quality at high magnification

The broad range of accessories available for use with these industrial zooms may tempt you to use one of them as the core of a relatively low-cost alternative to an expensive, high-magnification research microscope. However, I should warn you against this. These zooms are primarily designed for low magnification work, and any attempt to use them above roughly 50x is likely to yield disappointing results, especially if you intend to use demanding lighting systems like phase-contrast and DIC. This report from Sandia National Laboratories describes exactly such an attempt to use the Navitar Zoom 6000, which failed to provide an acceptable image quality.

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