When viewing transparent stereoscopic pairs presented in monolithic mounts, full enjoyment of the three-dimensional experience depends on the precise two-dimensional placement of the left and right film “chips.” Their placement not only controls the apparent location of the stereo “window,” but also affects how harmonious the stereo pair appears.

The location of the stereo window can be accurately controlled by simply moving the chips closer toward each other or further apart. But freedom from disconcerting “retinal rivalry” along the horizontal edges of the stereo window requires that homologous, i.e. similar, points in each chip of the pair be lined up horizontally. When lined up, visually important details near these edges will appear equally present in both the left and right source images (as long as the left and right optics are well-matched). During mounting, careful vertical adjustment of the left or right chip, up or down relative to its mate, can compensate for idiosyncrasies in the camera(s) or film path, and can reduce or eliminate this kind of retinal rivalry.

Especially when stereo slides are projected, mounting perfection is a desirable aim. The exact location of the film chips that create the stereoview should therefore be controlled to within about a twentieth of a millimeter, or about two thousandths of an inch. Unwanted discrepancies even this small can become noticeable, and possibly annoying, when images are highly magnified and viewed stereoscopically.

The precision required for projection-quality stereo mounting can be an obstacle for beginners. But if cost is not a determining factor, RBT mounts (adhering to the Realist 101 X 41 mm specification) offer many convenient features that make accurate mounting relatively easy.

RBT MOUNT PROS

• easy-to-use track and pinbar system
• good precision and durability
• reusable plastic construction
• five window sizes, 4 perf narrow to 33 mm wide
• accepted for reliable projection worldwide

RBT MOUNT CONS

• expensive (about 6X more than cardboard mounts)
• window edges are conspicuous in hand viewers
• occasional loose fit of pinbars, or window edge defects

The clever RBT system of asymmetrical pinbars that are a sliding fit in the upper and lower tracks of the mounts allows the left and right film chips to be placed anywhere along a precise X axis. Each pinbar has three raised nubbins that fit the sprocket holes of 35 mm films, so that the film chips can be held securely in place before pressing the two halves of the mount together.

The slight pinbar asymmetry allows the vertical offset between the left and right chips to be accurately adjusted. Made of white plastic, the standard pinbars that come with all RBT mounts could be called “05” pinbars, since they move individual chips up or down 0.05 millimeters. For the stereo pair, combinations of up or down pinbar orientation in the upper or lower tracks provide the standard 0.1 millimeter vertical adjustment increment and 0.4 millimeter adjustment range.

When 0.1 millimeter increments are too coarse to satisfactorily eliminate vertical discrepancies between the left and right chips, or when an offset greater than 0.4 millimeter is required, two kinds of alternate pinbars for RBT mounts can be employed. Both of these are made of a hard gray plastic, and could be called “10s” and “20s”, since they provide vertical displacements of 0.1 and 0.2 millimeters, respectively. They can be told apart by the “1.0” or “2.0” that appears on their front faces.

The principle of pinbar asymmetry and adjustment offsets that depend on the orientation and location of the pinbars is simple. But with three different pinbar types, upper and lower tracks and left and right sides, there are over 150 possible combinations, providing various amounts of left / right chip offset in increments of 0.05 millimeter.

As supplied, the alternate pinbar package contains 50 “10s”, 50 “20s”, and an explanatory sheet with several offset tables, in German. While it's easy to understand the information in these tables, English is my mother tongue, so I made a master table translation. I used “top” and “bottom” instead “upper” and “lower” to describe the tracks, so the difference is more noticeable. As with the tables supplied by RBT, combinations that only use the top track for both pinbars have a background color. This feature comes in handy when using RBT mount jigs for easy mounting in Spicer cardboard mounts, since only the top track can be used.

Here is just the table (in PDF format, file size ~100K). Depending on your browser plug-in setup, this may be displayed in the browser window. For printing, depending on your country settings or facilities, you may want to resize this table's native A4 page to your local letter size. Some printer software has a convenient “fit to page” checkbox. Get the free Acrobat reader if you don't already have a PDF file viewer.

Here is a Zip file archive of the table. It also is about 100K in size.

In North America, these useful alternate pinbars are available from 3-D Concepts. They are supplied in packages of 100, containing 50 each of the 1.0 mm and 2.0 mm varieties, at a cost of $7.00 per package, with price reductions based on quantity.