Friday, June 19, 2009

Mounting with RBT mounts - Part II

Vertical Adjustment

While most mounts rely on slack built into the mount for vertical adjustments, RBT mounts use a clever method that allows vertical adjustments in discrete steps.

If you look closely (Fig. 3), the pin bars are not symmetric but they have two small holes on one side (up or down, depending on how you orient the pin bar). The film chip is shifted by 0.05mm towards the side of these holes. In addition, the two channels in the RBT mounts are not equally distanced, but the top channel is placed 0.3mm higher (to the top edge) than the bottom channel.

If you place the pin bars in the same channel with the same orientation then the film chips will be aligned vertically (Fig. 4-1) By flipping the left pin to “look up”, the left film chip is raised by 0.1mm (Fig. 4-2). By placing the pin bars in opposite channels with the holes facing each other (Fig. 4-3) the left chip is raised by 0.2mm. Two more combinations are possible which lead to 0.3 and 0.4mm shifts (Fig. 4-4 and 4-5).

Since there are 4 ways to place each pin (2 channels + 2 orientations), there are 16 combinations for the pair of pin bars. Only 5 shifts are possible (0, 0.1-0.4mm). The rest of the combinations just shift the film chips slightly up or down. For example, putting the pin bars in the upper channel “facing up” pushes the film chips as far up as possible, while putting the film chips in the bottom channel “facing down” pushes the film chips as far down as possible, a difference of 0.4mm. This small shift might be useful in some cases.

The bottom line is that the RBT system offers vertical adjustments in 4 discrete steps of 0.1mm each. This system is particularly useful with certain stereo cameras in which one film chip is consistently higher or lower than the other (due to a slight offset of the lenses). I have seen this condition in many Stereo Realist cameras. In this case, if you know for example that the left chip is 0.1mm lower than the right chip, then you can always position the pin bars directly as shown in Fig. 4-2 and immediately get perfect alignment with RBT mounts while other mounts might require tedious adjustments with each mount.

The RBT vertical offset system is a bit slow (switching placement of the pin bars requires setting the stereo window from scratch) and has been criticized for lack of accuracy since adjustments smaller than 0.10mm are not possible. To answer this last point, RBT has introduced the so-called “0.10 Gray” pin bars, which when combined with the standard “0.05 White” pin bars they can produce adjustments in 0.05mm steps, and 0.5mm total shift. For those who want even greater range of shift, “0.20 Gray” pin bars are available, further increasing the total shift to 0.7mm.

For the majority of my work I have found that the standard white pin bars are adequate for accurate stereo mounting.

RBT Mount Sizes

The RBT system currently offers 7 different sizes of aperture openings (Fig. 4 - not all sizes are shown there). These are:

· RBT4 (21x16) for half-frame 4p slides (taken with a Nimslo for example).
· RBT MD (23x19.5) for Realist close-ups.
· RBT5 (23x21) for standard “Realist” 4p format slides.
· RBT 7 (23x28) for “European” 7p format slides.
· RBT 30 (23x30) for Horseman slides or cropping W, F images.
· RBT W (Wide, 31.5x23). This is the size used in certain RBT cameras.
· RBT F (Full frame, 33x23) for any standard camera (used for stereo on a slide bar or twin camera rigs) and certain (RBT usually) stereo cameras.

This variety is useful for cropping slides to improve composition. For example, slides taken with a FED stereo camera can be mounted in RBT 7 mounts without any cropping (a small amount of cropping is built into the size of the mount; for example 7p cameras produce 24x30mm images, cropped to 23x28mm in the mount) or in any of the previous 3 sizes (RBT4, MD, 5) for cropping.

Horizontal Adjustment

Horizontal adjustment in the RBT mounts is easily done by shifting the pin bars in their channels. Both pin bars can be shifted by the same amount for cropping, or one pin bar can be shifted with respect to the other for adjustment of the stereo window.

The issue of stereo window placement is discussed in a number of Tutorials in the Stereo Tutorial Volume. This is a fundamental issue in stereo mounting. What is unique about the RBT mounts is that this adjustment is easily done by shifting the pin bars without risking any twist or vertical misalignment.

Mounting with RBT mounts - Part I

A bit of History

The RBT mounts are made in Germany by Raumbildtechnik ( I actively sell these in my ebay store and also via my web site. Jon Golden of 3D Concepts has said that RBT developed these mounts to work well with their RBT101 Stereo Projector. The RBT mounts started appearing in the USA in the mid 1990s. The initial reaction was mixed. Some people loved them and actively promoted them, while others did not care for them. I remember when I was first faced with an RBT mount. My first reaction was that it was too thick. I also did not like the reflections on the edges of the mount, being used to the clean & sharp appearance (through the viewer) of aluminum mounts.

Back then most people were using cardboard mounts, or aluminum mounts with a cardboard foldover (for a viewer) or, for protection and projection, stereo aluminum mounts sandwiched in glass. But, gradually, glass fell out of favor and RBT mounts became more and more popular. Glass is heavy, it can break, and slides sandwiched in glass have not fared well with time. They often need to be remounted because humidity is trapped between the glass, the images are fading, or the tape used to bind the glass has become brittle and it is falling apart. Slides mounted in cardboard mounts have done much better with time, but cardboard is not the best choice for stereo projection.

Today most stereo slide enthusiasts favor the RBT mounts for stereo projection. As an example consider the PSA Stereo Sequence Exhibition. A few years ago there were quite a few sequences in cardboard or glass-mounted slides. Eventually all exhibition slides were mounted in RBT mounts. We are at a point were RBT mounts are strongly recommended or even required in certain stereo projection situations.

Basic Description

The RBT mount consist of two halves (one black and one white) that snap together, and a pair of pin bars that hold the film chips (Fig. 1). The mounts have two channels, one at the top and one at the bottom, to hold the pin bars. The pin bars are held with friction and can slide horizontally along the channel. Also, they are not symmetric and their exact orientation is used for vertical alignment (more about this later) Each pin bar has three “bumps” which fit into the sprocket holes of the film chips.

To mount in an RBT mount, follow these steps (Fig. 2, details will come later): 1) place the black half down, 2) put the pin bars in the upper channel of the mount, 3) place the film chips in the pin bars, making sure that they are centered in the openings of the mount, 4) place the white half at the top of the black and snap the two halves together.

Advantages of the RBT mounts

The RBT mounts have the following advantages:

1. Thick & rigid mount
2. No twist (rotation)
3. Controlled vertical adjustments
4. Easy horizontal adjustments
5. Wide selection of sizes
6. No tape/glue, reusable

The extra thickness and rigidity of the RBT mounts is a plus when it comes to stereo projection. These mounts are held well in place by most stereo projectors. Thinner cardboard mounts can warp or float in projection, resulting in focusing problems. RBT mounts do not have these problems.

Since the film chips are held by the pin bars, which ride along the same (or parallel) channels, it is impossible to twist the film chips during mounting. RBT mounts are unique in that respect. All other stereo mounts have some slack and twist is possible. Twisted film chips lead to rotation, a very serious mounting error which, unfortunately, is seen in many cardboard-mounted slides in projection.

Just for these two reasons I recommend RBT mounts for projected stereo slides, not only for beginners but for all our stereo club members.