Focusing racks are useful in close-up and macro photography. In these types of photography, it is common to set a magnification first (by choosing a focusing distance or reproduction ratio with the lens focusing ring), and subsequently to focus by moving the camera and lens back and forth as a whole. Small adjustments of the distance between camera and subject are best performed with a focusing rack.
A focusing rack has a fixed platform with one or more threaded holes for attachment to a tripod head, and a moving platform with an attachment for the camera (or lens foot) that slides along a prismatic metal bar or a pair of parallel metal rods. A rack-and-pinion gear or a screw mechanism allows the camera platform to move smoothly in small increments. Most bellows possess a comparable mechanism for moving the bellows, lens and camera as a whole. In some cases, a focusing jack can be used as an alternative to a focusing rack.
Since macro photography is my main area of photographic work, I expect a focusing rack to be sturdy, durable and reliable, and have little patience with equipment that hampers my work, rather than helping it. A focusing rack of high quality must satisfy several requirements:
Four models in my possession are shown and discussed below.
Unknown brand and model (above). This is mostly aluminium alloy and moderately heavy. The prismatic rail runs in a nylon insert of the platform. The platform has both 1/4 and 3/8 in. sockets. The large knob turns a metal gear that meshes with a plastic rack at the bottom of the rail. The small knobs locks the platform. Small screws hold the nylon insert in place, and can be used to adjust its pressure against the rail (thus controlling friction). Movement is smooth, but the plastic rack under the rail is reason to worry about premature wear, especially if the rack is mounted vertically. Locking still allows creep under a heavy load.
Velbon Super Mag Slider (above). This is a current model made mostly of magnesium alloy, and therefore very lightweight. It actually consists of two racks mounted at 90 degrees from each other. The two racks can be separated by using a supplied hex key. The larger platform can be used by itself as a conventional focusing rack. The shorter rack could be used in a similar way, if a few mechanical parts are added. Each platform runs on the rods on plastic bushings. Instead of a gear, both racks have a third, threaded metal rod. In the smaller platform, this rod is turned by a knob. In the larger platform, the threaded rod is fixed, and the knob turns a threaded nut (via a gear) that revolves around the threaded rod and moves the platform. There is no need for a locking knob, because this mechanism does not allow creeping of the platform under load. The movement of the platform is smooth, but it takes several revolutions of the knob to advance the platform by a small distance. This can be useful at high-magnification macrophotography, but is too slow at a 1:1 reproduction ratio or lower. As a whole, there is no wobble or play, but a slight flexibility of the whole construction that might invite vibration.
Pentax focusing rack (above). This is an old model, which I have owned for more than 25 years. It is an all-metal construction and relatively heavy. This is the sturdiest and most rigid (when locked) of the four models, and the only one of these with metal-against-metal sliding surfaces. The rods and gear are chrome-plated brass, while the casing is cast aluminium alloy. There are several provisions for fine adjustments of the alignment of rods and gear, mostly hidden under threaded metal caps that cover the adjustment mechanisms. Friction can be somewhat uneven and sticking may occur under heavy load, but lubrication with viscous fat and a careful adjustment of the rack can fix this. The small lever is a locking mechanism, but can be used also to control friction and eliminate residual play. Among these models, this and the following are the only ones that could be called a precision mechanism. It is certainly very durable, and the only one that somewhat approaches (but does not equal) in performance the focusing mechanisms of microscopes. In particular, some play when unlocked and a tendency to stick when used vertically with heavy cameras and lenses detract from its reliability.
Olympus OM focusing rack (above). This is
also an old model no longer manufactured, but now and then it appears on
eBay (unfortunately, most times at ludicrous prices). The top
platform was sold separately from a set containing the prismatic rail and bottom platform
(which are the same used in OM bellows). This may
make it difficult to acquire all necessary parts.
This otherwise very rigid and precise mechanism has two main weaknesses.
The first is that the racks are not rigidly mounted onto the rail. The rack is mounted onto springs that push
it outwards, and the screws that attach the racks to the rail are
designed to allow a movement of about 1 mm. The central portion of the racks also slightly bulges
outward. This is intentional, and presses the rack against the pinion to
take up any slack. Unfortunately, this construction allows the rack to slide
back and forth and sideways a fraction of a mm, which results in a dead play
of about half a mm when reversing the direction of rotation of the
focusing knob. If you are mechanically inclined, you can add a thin shim
under and along the side of the rack to correct this problem. Changing
the screws that attach the racks to the rail to ones with slightly larger heads also helps.
In time, this focusing rack became my favourite for studio close-up work with a tripod-mounted camera - until I came across the following one. The Olympus OM rack is still useful to me when I need a relatively portable focusing rack.
The above Zeiss focusing rack came to me as part of a second-hand, incomplete Zeiss epi-microscope. I did buy it because I was looking for a focusing rack for a Zeiss Tessovar, which it is designed to fit. In time, I also found a table-top focusing stand for the Tessovar, which is much more useful. Therefore, I resolved to modify an unused attachment ring for a Zeiss stereo microscope (cutting off most of the ring) to transform it into a focusing rack usable with camera equipment. The aluminium platform attached at the top allows mounting a camera, lens shoe, bellows or quick-release shoe system. This platform can be released from the rack with the small thumb-screw at the side of the mount, leaving it free for use with the Tessovar or other Zeiss equipment. Incidentally, I can mount the top platform of this rack also on the vertical column of the table-top Zeiss stand, which is useful for photomacrography.
This rack has a collar for adjusting tension (not visible in the above picture), and is extremely rigid and precise. The large, gray-painted support to which the nickel-plated brass prismatic slide and geared rack are attached is a 251 mm long solid aluminium bar. The black platform at the bottom is a large Manfrotto quick-release plate (dwarfed by the rack). This rack has a movement range of 172 mm.
There is simply no way to compare this focusing rack with those designed as photographic accessories that I have seen in my entire career. The Zeiss rack is extremely solid, extremely smooth, extremely precise and extremely versatile. It can easily hold any of the lenses I own, including a 300 mm f/2.8 (I don't really have a practical reason for doing so, but this rack can take it and more). With reasonable care, this type of focusing rack will last much longer than I do.
There is only one problem with this rack. It is extremely heavy, and putting it in my camera bag for using it in the field would effectively double the total weight of the macro equipment. It would also force me to carry along my heaviest tripod and head. In the studio, where weight is not a problem (and actually dampens any vibration), I am forced to use it atop my heaviest three-axes head and a heavy tripod. I would not risk using it on a ballhead, because its weight would force me to strain myself all the time to avoid the head and camera equipment from tipping in all directions whenever the head lock is released (and no ballhead would be able to carry this suddenly unbalanced load when unlocked). With a three-axes head, I only have to worry about one direction at a time.
Several other models of photographic focusing racks are available.
In conclusion, the quality of focusing racks sold as photography accessories is very variable, but never excellent (at least among the models I tried). There are flimsy ones that obviously are unsuitable for any serious use, and relatively good ones that are very expensive. The only focusing racks that I regard as excellent are the ones I salvaged from microscopes and optical equipment. If you have access to a metal workshop, you can transform one of these into a marvellous focusing rack. I have done this a few times, and often this does not require power tools, other than a bench drill.
This is a small collection of salvaged focusing racks, which I recovered from containers of discarded equipment at university institutions. Most are from microscopes, while the oversized one at the bottom comes from a large-format camera. Transforming any of these into a useful tool requires only the addition of an attachment for a tripod, and one for a camera. All these racks are made to hold heavy equipment in a vertical position, and therefore do not creep under load. They are also very rigid. Typically, they are made of brass and steel, and therefore very durable. In fact, all-metal microscope focusing mechanisms built in the mid-20th century are far superior to later ones, which often employ plastic inserts and even plastic gears.
Because of their weight, focusing racks built with salvaged equipment are most suitable for studio work. Most would be far too heavy to justify carrying in the field. Because of their extremely narrow mechanical tolerances, they are unbeatable for precision work, and will last literally a lifetime or longer. A general drawback of these racks is that typically they allow a range of movement limited to about 30-50 mm.
A microscope stage can also be very useful in high-magnification macrophotography in the studio. Oversized stages designed to hold large microscope slides, like the one at the top left above, are especially useful. You can insert a glass, metal or plastic plate onto the stage to hold the specimens to photograph, and use the micrometer controls to move it in two dimensions. The cost of new equipment of this type is prohibitive, but second-hand microscopes and microscope parts are common on eBay and other auction sites, and prices may be reasonable.
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