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.