Appendix A: Alternate Methods of Determining the Left and Right of a Stereo Pair

By far the easiest method of identifying the left and right pairs of stereo slides is to sort them while cutting. However, if you get confused, or your spouse mixes up your orderly pile of slides (don't laugh!) you can use one of these methods to sort them out again:

1. Often the easiest way to distinguish left from right is by the frame numbers.  Most 35mm stereo cameras have the take up spool on the left, meaning a lower-numbered frame is always the right- side image.  If the take up spool is on the left (e.g., Realist 45), the lower numbered frame will be the left- side image.
2. The "witness mark" that the camera places on the exposure can also distinguish between the left and right frames. If you examine your film chips, you will see that one has a notch or other marking somewhere along the edge of the exposure. Often this will be a half circle. If you look in the back of your camera at the two openings used to expose the film, you will see this notch on one of them. This identifies the left (or right) of each pair.
3. You can also inspect the film chips to see which is the left and which is right. Set them in front of you, with the photos looking "normal" (i.e. right side up, and correctly left-right). If you look at the left side of each film chip, the film chip which is the right one will have more image on it's left side (and vice versa for the left chip - it will have more image on it's right side.)  Note: this only applies to images from stereo cameras with a built-in stereo window.  It does not apply to images from twin camera rigs.
4. Use a pair of magnifying "reading" glasses (available at a local drug store). Using these you can "free-view" the film chips before mounting. Slides which have their left-right pairs correct will show a proper 3D scene, while those with left and right reversed will show "pseudoscopy" (i.e. near things will look far and far near). Try this out on a pair you know to be correct first to see the difference.
5. Some people like to use a slide mount to build a "slip-in" mount for sorting purposes. This is similar to the last method above, except that instead of using reading glasses, you tape a normal cardboard mount shut, cutting off the top half of the front, to create a mount which allows you to easily slip the film chips in and out. Then you can put both film chips in, drop it into a viewer, and see quickly whether or not the scene looks "normal" or "pseudoscopic".