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.
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
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
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
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.
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
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
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
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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