Friday, June 19, 2009

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.


Anonymous said...

yeesh- i have been using these for a little while and had no idea the pins were not symmetric - good thing I read this article! now I have to go back and check to vertical alignment of my slides..

Barry Young said...

All the fog has now cleared. I can clearly see why RBT mounts are superior. Thank you for this concise well written article.