iShoot Macro Focusing Rail 150
wormscrew stage for macrophotography
Wormscrew stages and racks are named after the mechanism used to move their sliding platform. These stages are one-dimensional, with the platform typically sliding on prismatic supports in the simplest and cheapest implementations, or, when durability must be high, on linear ball-bearings. When precision is the main requirement, metal-against-metal prismatic rails are preferred, e.g. in microscope focusing blocks. Linear ball-bearings are often used in industrial equipment where tolerances are less stringent but friction must be kept low.
The large majority of focusing stages designed for photographic use are, unfortunately, low-cost products of decidedly poorer quality than microscope focusing stages and industrial micrometric stages. These photographic stages often display a multitude of problems including loose fit of moving parts, flexing, and uneven movement (alternately "sticking and jumping"). This makes these stages essentially useless for any serious applications.
The wormscrew stage tested on this page is a relatively recent introduction by the Chinese brand iShoot on eBay, apparently modeled on more expensive wormscrew stages manufactured, e.g., by Novoflex, RSS and Sunwayfoto. It is marked as "Macro Focusing Rail 150", where 150 indicates that the rail is 150 mm long. The actual movement of the platform along the rail is about 115 mm.
I don't have the RSS stage (or any equivalent highly priced photographic focusing stage) to compare this stage with, because their purchase does not make economic sense to me. For a similar or lower price, I can purchase a second-hand microscope focusing rack or industrial linear stage of better mechanical quality and higher durability, and the higher weight of the these devices is not a problem for laboratory use. However, the price of this iShoot wormscrew stage is low enough to make it an interesting alternative if it proves to work well enough for field work, without involving too much of a financial risk if it does not.
This wormscrew stage looks well-manufactured. The base is an integral, long Arca-compatible plate, complete of safety bolts that prevent the plate from sliding off a clamp when the latter is slightly opened. To completely extract the plate, the clamp must be opened significantly wider. The metal is blackened, with silk-screened scales and markings on the platform and laser-etched scales on the rails (where friction would wear out silkscreened scales).
The clamp on the moving platform is large and massive, and oriented perpendicularly to the bottom plate. The two sliding rails are likewise massive and spaced significantly more apart than the edges of an Arca plate. This makes sideways wobbling less likely. The platform moves smoothly and silently along its rails - too smoothly for a non-lubricated metal-against-metal mechanism. There must be nylon pads hidden under the platform, which should last many years of moderately frequent use with a limited weight attached to the platform. The pads appear to be sufficiently well-made to prevent any significant wobble in the macrophotography range up to 1x magnification (although only tests, rather than visual examination, can detect wobble at higher magnification).
The wormscrew is well-machined, with none of the surface roughness typical of stock threaded rods. Knobs of different diameters are attached at either end of the wormscrew. There is no detectable loose play of the threaded rod within its end bushings (either longitudinal or sideways). These are basic requirements of any macrophotography stage.
It takes some effort to rotate the knob of the wormscrew. The threaded nut that moves the platform against the wormscrew is, functionally, less than a half-nut. It is not shaped as a nut (or half-nut), and is instead a shaped piece of steel that fits in a slot under the platform, where it can be moved by a control located on one side of the platform. It is convenient to continue calling this part as half-nut. It disengages from the wormscrew by pushing in the side control. A small metal spring pushes the half-nut back into contact with the screw when the control is released. The same control can also be twisted slightly to lock the half-nut against the screw and prevent disengaging.
A support in the middle of the stage prevents deformation of the threaded rod under pressure by the half nut. A nylon bushing protects the thread from direct contact against the metal of the rail. This detail suggests that the design was modified at some stage during testing, to correct for a problem caused by the length and flexibility of the threaded rod. Although there is some obvious concern about this bushing wearing off by scraping agaist the thread, it is nice to see they tried to solve a problem.
A thumbscrew on the opposite side of the platform is used to lock the platform against its rails.
The operation of a focusing stage, rather than its looks, is the only thing that really counts. I mounted this stage vertically on a photomacrography stand equipped with an Arca-compatible clamp, and placed an Olympus E-M1 Mark II camera equipped with an RSS bottom plate in the clamp of the moving platform.
With a relatively heavy macro lens attached to the camera, problems became immediately apparent. I chose a vertical orientation because it is more demanding of a focusing stage under load, besides being the normal orientation I use with focusing stages. A camera and lens mounted on the platform result in an oblique force applied by the platform on its rails, not just an increased vertical load. The observed tendency of the platform to alternately seize-and-jump may not be a hopeless problem. Lubrication of the rails (which I did not attempt) may improve the sliding movement to the point of making it sufficiently smooth.
The show-stopper is instead a backlash of almost one millimeter of the platform, which combines with the seize-and-jump effect to make precision focusing impossible. This backlash cannot be eliminated by loading the platform, e.g. with a rubber band or spring (which I tried, unsuccessfully), because it is combined with an excessive friction of the platform against the rails, which increases dramatically with oblique loading. The half-nut is simply too short for its slot at the bottom of the platform, and moves and twists back and forth in this slot.
I discovered that the amount of backlash can be somewhat reduced (to about half a mm) by tightening the control that prevents the half-nut from disengaging the threaded rod. Still, the remaining amount of backlash is too much for my liking. The above animation shows the remaining amount of backlash after locking the half-nut.
No amount of lubrication can solve this problem. There is not enough space to add shims to the half-nut to improve its fit, and no obvious way to permanently fix the shims. Given the shape of the half-nut and its loose fitting on the spindle of the engagement control, shims would probably not be effective anyway. In theory, it may be possible to discard the half-nut and its disengagement control and replace them with a permanently engaged nut tightly fitting in the slot under the platform, but machining a one-of-a-kind nut is hardly worth the effort. Backlash of the new nut against the threaded rod (which is not a precision part, either) may become the next problem. It is safer to give up on this stage and use instead an industrial-quality linear stage equipped with a micrometer.
Example: a high-quality industrial linear stage
The illustrated model works very well, and is of higher-than-average quality. It is made from machined stainless steel for high stiffness, and has double rows of ball-bearings for increased load-bearing capabilities (enough to carry my own weight). It is therefore both heavy and expensive. For many applications in macrophotography, a cheaper and lighter micrometer stage with body machined from aluminium alloy and single rows of ball-bearings is fully adequate.
I added an Arca-compatible plate on one side of this linear stage (via a plain aluminium plate because the holes on the plate did not line-up with available screw sockets on the stage) and an Arca-compatible clamp on the opposite side, so it is now equivalent in connectivity to the wormscrew stage discussed above. Most industrial stages of this type have multiple threaded screw sockets for attaching to other equipment, but an intermediate plate is often needed to attach photographic equipment. This is easier and safer than drilling and tapping new holes in a linear stage.
These industrial stages must be loaded, i.e., either mounted vertically with the micrometer tip pushing up the platform, or, if mounted horizontally, a spring or rubber band must continuously push the platform against the tip of the micrometer. If properly done, there is no detectable backlash. Many stages, including the illustrated Newport model, have built-in springs that do this. A strategically placed rubber string is usually an easy fix for those that do not.
The travel length of the platform of these stages is relatively short. The most common compromise between travel length and cost is probably 25 mm, with 51 mm being a close second. For use in close-up photography and macrophotography, the stage should be combined with a longer Arca-compatible rail and clamp to allow a quick or larger change of distance from the subject. Relatively cheap Arca-compatible rails of good quality made in China are currently available in lengths up to 400 mm. With most micrometers used on these stages, practical focus steps of 10 µm are easily obtained. The use of these stages therefore extends well into photomacrography.
Microscope focusing racks with coarse and fine controls are even more precise than industrial linear stages. While industrial linear stages usually can be used also in the field, however, microscope focusing racks are normally used only in the laboratory/studio.
After publishing this page, I found out that Really Right Stuff makes a wormscrew stage very similar to the iShoot stage reviewed on this page. In fact, the two are too similar to be just a coincidence, and I regard the iShoot stage as a clone of the RSS B150-B. I don't have the RSS stage to test, and therefore I cannot state how well it works, and whether it has the same fatal flaw as the iShoot stage. The main visible differences between the two products are the red color of the knobs in the iShoot product, the lever-locked Arca clamp of the RSS product, and machining details of the knob that locks the half-nut. I have not found pictures of the RSS product that show details of the locking mechanism (the little I can see looks identical in the two products), and therefore I cannot say anything about its tolerances.
Reviews of this product on the RSS web site are generally positive but mixed, with a few reviewers mentioning eccessive backlash, sideways shifting when locking the platform, non-smooth turning of the wormscrew, and other loose tolerances.
The RSS stage costs roughly 8 times more than the iShoot. At this price, industrial micrometer-operated stages with far better mechanical tolerances are directly competitive even when bought new (these items are continuously available used on eBay, at a fraction of the price). It might be a better idea for RSS to source an industrial stage from Newport and add to it an Arca-compatible plate and clamp, than try and make a complete rail.
At first sight, the iShoot wormscrew stage is reasonably well-manufactured, with a good surface finish, acceptable stiffness and acceptably good fit of the rotating and sliding parts. Its nylon bushings tend to catch when obliquely loaded, but this might be eased by lubrication. All this, however, is made useless by a single poorly designed part (the half-nut that engages the wormscrew) that causes a backlash of about 1 mm, far too much for the intended use in macrophotography.
Opinions of the acceptability of this defect may vary, see for example this thread on photomacrography.net and other posts on that site linked from said thread. My own position is that the loss of usability caused by this defect cannot be compensated by a low price - using a fiddly and unreliable gadget is always going to be a frustrating experience, regardless of how cheap it sells. In this particular case, a better design with lower mechanical tolerances would probably cost very little to the manufacturer, which strengthens my inclination not to recommend this product.
If you want a reliable focusing stage for macrophotography and photomacrography, forget the iShoot wormscrew stage. Buy instead an industrial micrometer-operated linear stage, or a suitable microscope focusing rack.