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Mounting Stereo Cards

The Basics    The Stereo Window    Practical Help

by David Lee 

This paper is intended as a guide for those who would like to make stereo cards, but are unsure exactly what to do. In order to make things easier to follow, this paper is divided into 3 sections, so that the reader can choose the level of involvement that reflects his/her interest without having to be confused by unwanted information. The first section is titled The Basics for those who want to know the most elementary details only. The second section is The Stereo Window for a more detailed account of trimming and mounting. The third section, titled Practical Help in Mounting a Stereo Card, tells how to make jigs which assist in quick and accurate trimming.

The Basics

Generally, the size of a Holmes stereo card is 3" x 7". On figure 1 these are dimensions F and G respectively. While the nominal height of the card is 3", this dimension could range from less than that to 4" or even more, but it would get progressively more difficult to see the top if it were more than 4". While the nominal width of the card is 7", slightly less, say 1/32" to 1/16" works better because the holder for some viewers is exactly 7" and a 7" card tends to bind. The corners can be left square or they can be cut round. As for the material, a 4 ply acid-free mat board is preferable. Several companies make them, including Bainbridge, Crescent, Rising, and Strathmore. Virtually any color can be found in acid-free board, but I would suggest a color which is less saturated than the colors in the image, so that it does not draw ones eye from the image.

The height of the image (E) is generally about 3", but it could be more or less. If the height of the card (F) were 3" and the height of the image (E) were 3" this would leave " to be split between I and J. I prefer leaving " for each, but that is an aesthetic decision.

The space between the two halves (C) varies between no space for most of the antique cards to about 1/8" or even more in some cases. One-sixteenth inch seems about right to me.

The width of each half (H) should not be more than about 3". The exact maximum width , though, is based on the infinity separation which will be considered in the next section.

Figure 1


The Stereo Window

The concept of the stereo window is that the image will appear as if through a window, with everything in the scene behind the window. In order to understand this concept one must first realize that as the scene recedes in depth the corresponding points get farther apart. The corresponding points for the near point of the scene are the closest together, while the corresponding points for the far point of the scene are the farthest apart. Therefore it follows that the distance between the edges of the window (D) should be equal to, or slightly less than, the distance between the near corresponding points (B). This is the commonly accepted practice with modern stereo images. The makers of antique cards seem to have had no concern for the stereo window and consequently it is usually somewhere in the middle depth of the scene and occasionally at infinity.

As was mentioned in the previous paragraph, the corresponding points of the far point in the scene (often called the infinity point) have the greatest separation and the maximum infinity separation (A) is determined by the lenses of the stereoscope. The maximum infinity separation for the stereoscopes I measured ranged from 33/8" to 35/8". Some individuals are able to view images which have greater infinity separations than that, but the average person will have trouble. For this reason it is prudent to limit the infinity separation to 33/8" (H).

A final point which may be of interest to the technically minded, especially those who do not use stereo cameras. The total deviation (TD) of the image is determined by subtracting the distance between the near points (B) from the distance between the far points (A). That is A - B = TD. If TD is too small the scene will appear flat. This would be the case when using a stereo camera with a distant scene. If TD is too large, there is too much depth and it will create eye-strain or make the image difficult to view. For best results TD will probably be in the range of 1/8" to 3/16".

Practical Help in Mounting a Stereo Card

After one has made a stereo pair of the right size it is ready to be trimmed and mounted. The first step is to trim the bottoms so that the bottom edge is in exactly the same relative position on both halves. From that the top can be trimmed by measuring an equal distance.

If one is mounting images made from a stereo camera, such as the Stereo Realist, then trimming the sides need not be an overly critical matter. The sides can simply be trimmed off evenly.

If you are using a regular camera (or cameras) then you will have to trim more off the right side of the right image and the left side of the left image. If you want to be more exacting, you can follow these instructions for trimming the sides. First trim the right side of the left image, probably close to the edge, but wherever you want. Next locate the near point in the scene and measure from it to the edge you just rimmed. Locate the same point in the right image and trim the right edge the same distance from the near point as in the left image. Next trim the left edge of the right image, probably close to the edge. Measure the width of the right image and trim the left image the same width.

This method works reasonably well for making several stereo cards, but if you make more than that and you want to do it accurately and efficiently, then it helps to use jigs. It works well to have three jigs altogether.

The first jig (see figure 2) is for trimming the tops and bottoms. Cut a piece of glass or plexiglas 3" high (or whatever height you have chosen for your images) and about one foot long. Make sure it is exactly the same height from one end to the other. It is also helpful to make scored lines along the length of the glass at whatever intervals you chose, but they must be parallel to the edges. Then cut the glass in half, so you have 2 pieces about 6 " long (A). Cut pieces of mat board (B) bigger than the glass and glue a small piece of mat board (C), the height of the glass, to the bigger piece and tape (D) the glass to it so that the end of the glass can be lifted and the image placed under it. You should also glue small pieces of mat board (D) to the bigger piece at the corners of the glass, so that the glass will not slip. Finally, put a piece of tape at the end of the glass so it will be easier to lift. To trim the tops and bottoms simply place each image half under its piece of glass and maneuver them until they are in the same relative positions with the edges (or the parallel lines) of the glass. With a razor blade, trim the top and bottom edges carefully.

Figure 2

To trim the sides you will need another jig (see figure 3). Cut a piece of glass (A) about 3" wide (the exact width will depend on the width your images will normally be, mine are 215/16") by about 6" long. You will tape (D) it to a piece of mat board (C) in a similar manner to that of the previous jig, again with reinforcements (E) on the corners. The major difference is that you need a method of trimming the image exactly perpendicular to the top and bottom edges. To do this you will glue a piece of photographic paper (F) in the manner shown in figure 3, making sure the edge of the photographic paper is exactly perpendicular to the edge of the glass. To trim the images, place the left one under the glass so that the bottom is against the photo paper and the right edge is positioned where you want to trim it. With an ultra fine-point Sharpie pen make a mark at the near point. Then trim the edges with a razor blade. Next place the right image under the glass so that its bottom is against the photo paper and its near point is under the pen mark. Trim the edges. The window will thus be placed at the near point.

Now mount the image. First one must consider the adhesive. One could use dry mount tissue (recommended for fiber based prints), glue, or Positionable Mounting Adhesive (by 3M), which I recommend for RC prints (color or black and white), although it will work for fiber based paper also The adhesive (except for glue) should be applied before trimming. In order to position the images you could make pencil marks on the mount or you could make a jig to help you quickly position it.

Figure 3


In order to make a jig (see figure 4), start with a piece of mat board (A) about 8" x 10 ". Glue another piece of mat board (B) about 1" wide to the right side. Glue another piece (C) about 2" wide to the bottom. Glue a 1" piece (D) on top of C and tape (E) another piece (F) to D. The width of F should be such that it extends over C exactly " (or whatever amount you have decided to use as the distance between the bottom of the image and the edge of the card). Finally, measure from the edge of B exactly 3" and make marks (G) on either side of that point 1/32" away from it (or whatever you choose to be the separation between the 2 halves.

To mount an image, take a piece of card stock and place it against the edges of C and B and under the edge of F. Determine which is the left image and which is the right image. Take the left image and peel off the backing paper (if you are using Positionable Mounting Adhesive) and place it on the card with the bottom of the image against the edge of F and the right edge at the left side of G. Do the same with the right image but place it so that its left edge is at the right side of G. If you are using drymount tissue you can trim off the top part of A so that the card hangs over the edge by about ". Place one image and hold it in place using a wooden clothespin. Do the same for a the other half. Place the bottom half of the card in the drymount press to tack the images in place. Then open the press and put the card all the way in to adhere it.-

Figure 4

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