Zeiss TessovarThe Carl Zeiss Tessovar is a workhorse of macro photography and photomacrography. It is still frequently used in academic institutions, and often mentioned in the "Methods" section of scientific papers, especially in biomedical and natural science fields. It is also used in forensic examination. I have failed to find first-hand information about the period in which my specimen was produced, and the equipment and literature in my possession carry no dates. Early versions with two objectives may have been produced in West Germany in the 60s and (more likely) 70s, in the heyday of macro photography. A more modern version with a four-objective turret, like the one discussed below, may date from the 80s. I was able to buy a second-hand Tessovar on eBay, practically unused except for a potent cigar smell that would have been the pride of any well-used Victorian ash-tray. Like most units available on the second-hand market, it lacked essential accessories, including a focusing rack. Fortunately, most of the Tessovar accessories are shared with Zeiss stereo- and/or photo-microscopes, and therefore not impossibly rare or expensive. If you are interested in a reasonably complete and well-kept Tessovar at a reasonable price, be prepared for a long wait. As a whole, I have seen no more than 5-6 units in working conditions advertised on eBay in one year, with unreasonably high prices asked in over half of these cases. Some of these units seem to have remained continuously advertised on eBay for at least 20 years. Comparison with other lensesThe Tessovar is unlike any other macro and photomacrographic lens and, in external appearance, more like a stereo microscope than anything else. However, it does not use a composite optical system, and therefore it is not a microscope. Also, it does not provide stereo images. It is, instead, a parfocal zoom lens optimized for close-up, macro photography and photomacrography in the magnification range 0.4x-12.8x (when used with 35 mm film and the standard set of matched auxiliary lenses). The only other legacy zoom lenses that come close to the Tessovar are the extraordinarily rare Zeiss Zoom Luminar, optimized for magnifications between 2.5x and 5x, and the Wild photomacrography zooms. In the close-up range, the Micro Nikkor 70-180 mm focuses from infinity to 0.77x and is parfocal (i.e., it does not need refocusing when zooming). I also experimented with the Schneider Betavaron, which was originally designed as a zoom lens for darkroom enlargers, but does an acceptable job in close-up photography when a parfocal zoom is needed. Its magnification range is 0.1x-0.33x, i.e., it almost perfectly complements the range of the Tessovar. Among more modern equipment, there are a few industrial macro zooms, made for example by Unitron , Navitar and Optem, that offer a comparable magnification range. My general impression of these devices is that they are designed for videocameras with small sensor and a relatively low resolution. They can be used for digital imaging with modern cameras, if one trades ease of use for a less-than-stellar image quality (in part because of the low NA of these devices, and in part because they are simply not designed for high-resolution imaging), but most of these zooms do vignette already on APS-C sensors. Optical adapters for larger sensors are available from the makers of these zooms, but are of less than impressive quality. Construction and useThe Tessovar name implies that it uses a modified Tessar design (which was one of the favorite optical formulas employed by many manufacturers, before computer-designed lenses became the norm), and a variable focal length. It was at the center of a line of Zeiss accessories designed mainly for use in a laboratory environment. 
The base unit of the Tessovar system (i.e., the barrel containing the optics and the associated controls), stripped of all accessories, is a mostly solid chunk of metal with very small optical elements, and weighs almost 1.8 kg. It feels like a tank, and would do serious damage to a stone floor if dropped. It has two knobs to operate the zoom. They are attached to the same axle, so you can operate either one with either hand at your convenience. Their purpose is also to carry four engraved scales. The base unit also has an aperture ring. These controls do not work in the manner you would expect for a lens, and are discussed below. All the Tessovars I have seen are painted in a light gray enamel (sometimes yellowed and faded) used also for Zeiss microscopes of the same period. Possibly, some early Tessovars are available in medium-gray hammered enamel. Most camera adapters for the Tessovar are instead black. 
Several Zeiss microscope parts and photo tubes for Zeiss microscopes can be mounted onto the Tessovar, because their bayonets are identical. I do not know exactly which of these mechanically compatible accessories are also optically compatible with the Tessovar (a common problem with Zeiss microscope parts is that most of them can physically be assembled together, but many of these mechanically possible combinations do not actually work or make sense). As a general rule, photo tubes and photo adapters for the Tessovar and 35 mm cameras have no internal lenses (they may have prisms and beam splitters). I have been able to test satisfactorily a few microscope parts by removing their internal lenses (which are necessary with microscopes in order not to change the optical tube length), like the beam-splitter assembly mounted on top of the Tessovar in the above picture. The photo tube mounted at the top of the beam splitter contains both a photo ocular and a set of relay lenses (both are essential to its function, and must not be removed). The above picture also shows the table-top focusing stand (extremely useful) and an illuminator (indispensable for focusing) and its transformer. A fully configured Tessovar becomes quite a big, complex and heavy piece of equipment. C-mount adapters, large-format cameras and Polaroid backs for the Tessovar also usually have internal optics. 
The base unit has an attachment for a proprietary focusing rack. The user manual lists two types of focusing racks. One is similar to the focusing rack shown in the above picture (which, however, is probably a heavier and sturdier type, available only on special order). The other is short and made for attaching to a swing-out arm. There is also a base with integral vertical column and focusing rack (shown in an earlier picture, above). These accessories are the same employed by Zeiss stereo microscopes of the same period. The focusing rack shown in the above picture is a massive aluminum rod with threaded holes for standard 3/8" tripod screws at its bottom. These racks have a friction adjustment for the focusing knobs, and therefore can be used vertically or in any other position. There is also a very large table-top stand with a long rail that allows the Tessovar (or other types of photomacrographic equipment) to be moved up and down for a considerable distance. The frequent use of the Tessovar mounted vertically on a microscope stand is reflected in the fact that all scales and engravings are oriented properly when the front lens element points downward and the operator sits in front of the equipment. In practice, the orientation of markings is the same as in enlarger lenses, and the opposite of that found in general-purpose camera lenses. Although some pictures in the Zeiss literature show a Tessovar and focusing rack attached to an uncomfortably light-looking tripod head, you will need a very sturdy tripod and head capable of carrying several kilograms of gear without bending or creeping. The attachment for the focusing rack connects to a circular collar that embraces solidly the base unit. The small black knob on the collar (above picture) unlocks it and allows the base unit to rotate (but not to be extracted from the collar, which would require some disassembly of the base unit). This is not very useful, because the focusing rack attachment collides with the zoom knobs after a short rotation of the collar. The bottom portion of the base unit also carries two metal rings with attachments for the articulated arms of standard Zeiss stereo microscope lamps. These rings can be rotated freely once a a black plastic ring with a knurled outline is slightly unscrewed. The front optical group of the lens moves in and out within the front (=lowermost) part of the barrel when zooming (like in the Leitz Zoom Luminar), but never protrudes outwards. The base unit (without add-on lenses) allows a magnification range between 1.6x and 6.4x, and a constant working distance of 75 mm (i.e., the zoom is parfocal when used with a camera attachment of the correct type and length). As far as I can determine, the proper connecting tube between the Tessovar and a Nikon body should be about 69 mm long. I don't have a combination of tubes of this exact length, so my estimate might be off by up to a couple of mm. With a DSLR, you can use a significantly shorter connecting tube, and get away with it without vignetting. You also get a lower magnification factor and a brighter image in the viewfinder, which may be advantageous. Of course, you loose parfocality, so you need quite a bit of re-focusing when zooming. You may also use a longer tube and get a higher magnification. Although this means that the Tessovar will not be working at its optimal design parameters, it is of such good quality that you can change the tube length quite a bit without getting any noticeable degradation of the image. I find parfocality (or almost-parfocality) quite useful, so I prefer to use a tube length close to the design parameters. As a photo tube for my cameras, I modified a Zeiss microscope adapter for the Icarex camera, which contains no optics (it does have a rectangular internal light baffle, which I left in place to eliminate stray light). This modified tube is 44 mm long, which with the addition of PK-11 and PK-12 rings almost reaches the length necessary for parfocality (a small amount of refocusing is still necessary after zooming). The 44 mm ring can be used alone without vignetting on a DSLR, and probably even shorter tubes could be used. 
The above picture shows a (perhaps custom-made, or special-order) handle fitted onto one of the zoom knobs. It does make the knob easier to operate. A revolving nosepiece with four openings (above picture) attaches to a standard round Zeiss microscope bayonet at the front of the lens. One of the apertures is empty, while the others carry specially matched auxiliary lenses (0.25x, 0.5x and 2x) that modify the focal length of the base unit. This allows a total magnification range of 0.4x to 12.8x in four partially overlapping zoom ranges, with the additional parfocal working distances in each range of 320, 150 and 36 mm. All four ranges are labeled and color-coded on the zoom knobs. Corresponding color markings are present on the nosepiece. My unit came with an additional, unmounted achromat lens that was tentatively identified by the seller as a 0.4x auxiliary lens. It does mount into the revolving nosepiece in place of the 0.5x lens, but a 0.4x auxiliary lens is not mentioned in the documentation. It provides a zoom range (0.64x to 2.56x) largely overlapping the one of the 0.5x lens, and therefore it might have been an item available only on special request. 
Unlike what stated on some web sites and in some books, the revolving turret does not use standard RMS threads for the lens mounts, so you cannot attach a microscope objective here. The aperture ring is marked in 1 (open) to 4 (closed) arbitrary units and has no click-stops. A further peculiarity of the Tessovar is that the zoom knobs and aperture ring are interlocked, so that zooming also adjusts the aperture in order to maintain a constant exposure time. This was a great help with external light meters and fixed-power electronic flashes, but is of little practical importance with a DSLR and iTTL flash. In any case, the aperture ring can be turned manually to a different setting after zooming. In practice, the diaphragm should be fully open when focusing. It is stated in the literature, however, that the diaphragm opening as set by the interlocking mechanism is the optimal one to provide the best compromise between resolution and depth-of-field for a given magnification, so you should use this opening unless you need to obtain, for instance, the highest possible depth-of-field at the expense of resolution. As with most specialized macro photographic equipment, knowledge of the effective values of focal length and aperture are of little usefulness for the practical user, and the focal length is not even mentioned in the data table. It can be estimated that the effective focal length is roughly the same as, or slightly more than, the working distance. Thus, the focal length without add-on lenses should be about 75-80 mm. More useful data like the depth-of-field are documented. At 1x magnification, the maximum depth-of-field with diaphragm fully closed (effectively f/90) is 16.4 mm, which is quite remarkable, and far more than the one provided by general-purpose macro lenses (usually f/22 to f/32). Naturally, this implies a loss of resolution and contrast due to diffraction - there is simply no way around the laws of optics (short of using deconvolution algorithms and exotic constructions like the ones used to push the magnification of top-of-the-line modern microscopes beyond these limits). Nonetheless, there are occasions in which a relatively low resolution and a high depth-of-focus are preferable to a higher resolution and a partly out-of-focus picture. From this table, it also becomes apparent that the Tessovar was designed primarily for use with 35 mm camera bodies (and therefore, indirectly, with full frame or APS-C DSLRs and mirrorless cameras ). Adapters for other formats were available, but they contain optical groups to cover their film areas, and the magnification scales on the base unit no longer apply. My unit came with a C mount for 16 mm or 8 mm movie cameras. This adapter is mounted on top of an 80 mm long Zeiss extension tube. The adapter carrying the C mount has no markings, and there is no mention of a C mount in the accompanying literature. This adapter contains optical elements with a slightly diverging effect (roughly -1 to -2 diopters), probably to compensate for the smaller image circle and different registration distance. A Tessovar is definitely not designed to be carried in a camera bag. Although a "bare" Tessovar with camera adapter is not much bigger than a 200 mm macro lens, it is at least twice as heavy, and cannot be used without a precision focusing rack. The weight including a focusing rack, beam splitter unit, eyepiece and photo tube can easily exceed 5 kg.
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Focusing
