International Stereoscopic Union

By Bob Aldridge

If there was ever a subject that caused more pain and anguish to stereo photographers, it has to be mounting! And yet, once the concept "clicks" it really isn't that hard.

So, what is the basic concept?

In essence, the objective is to present the stereoscopic pair of images to the viewer in such a manner that their brain reconstructs the  (3D) scene in an effortless and natural manner. There are a few requirements that need to be met for this to happen.

First of all, and probably most important is the fact that when we look directly at an object, is is extremely unlikely that the lines of sight from our two eyes will not be parallel in the vertical direction. One eye will never have to look up whilst the other is looking down.

So, we must ensure that the points in the two slides representing the same point in space (the technical term is homologue) must be at the same height in the mount apertures.

The next requirement for correct mounting if the slide is to look "right" is that the scene must be correctly positioned in relation to the stereo window.

And this is where there is the biggest comprehension gap! What, exactly, is the stereo window?

Imagine that you are looking out at the world from your home through a normal window which is about 2 metres (6 feet) from you. Just outside there is a tree.

Now, if you move to your right, there will come a point where the tree is masked from your right eye by the window frame, but will still be visible to your left eye.

Move to your left, and the opposite will be true. You will see Less on the Left with your Left eye.

And that is the basis of correct mounting!

You see, when you are mounting film "chips" from a stereo camera it is possible to position the chips so that more of an object beyond the window can be seen by the left eye on the left of the scene. But this is contrary to the concept outlined above, using the real window. The scene will not look natural. It will appear to have grey, indistinct borders on the left and the right. If you increase the separation of the chips there will come a point where the condition is met and the edges will now look sharp and strong.

The trick, of course, is learning to see when the slide is correctly mounted!

For that a few tools might be useful. But first, let's look at a few examples of mounts that are currently available.

Mounts for Stereo Slides

There are two distinct mounting systems in use today:

The "Realist" mount systemThe 2 x 50 x 50 System (also known as 2 x 2 x 2 after the imperial measurements)The first was developed from the 45 x 107 mm glass slide system by Seton Rochwite to be a part of his Stereo Realist 35 mm camera system. The basic overall mount size is 1 5/5 x 4 inches (which has become translated to 41 x 101 mm) and has two apertures - one at each end, separated by an approximation for normal eye separation. The original aperture size for the Stereo Realist camera was 24 mm high by 23 mm wide, but a range of other apertures have been developed for use with other camera systems.

The second was derived from the standard 50 x 50 mm (2" x 2") slide mounts used by monoscopic photographers. For stereoscopic applications, a pair of mounts are used - one for each eye's view.

The "SAM" or Mounting Jig

Over the years there have been numerous designs of "Mounting Jig". The manufacturers of the "Hawk" range of stereo slide projectors were still selling a 20 year old design in the early 1980s...

Recently, a new range has been produced with "adapter plates" for various types of slide mounts. Somehow it acquired the name "SAM" which I am reliably informed stands for "Stereo Active Mounter" which describes the purpose of these devices very well because the stereo-photographer is able to see their slide in stereo through the lenses on the unit whilst moving the chips. Thus the effect of the movement  is instantly seen in three dimensions!There is nothing complicated about a mounting jig. All that is needed is a light source and a means of holding the slide mount under a pair of magnifying lenses, which should be slightly longer focal length than would be appropriate for a slide viewer to enable the chips to be easily manipulated.

Many of the classic mounting jigs also incorporated guillotines to cut the film.

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