Answers to The Stereo Quiz

	WRONG	WRONG		RIGHT	RIGHT
1

Discussion:

This picture is “flat as a pancake!” On purpose I have reproduced the same member of the stereo pair twice. So the two pictures are identical. Shown in the right is the correct stereo pair. If you study this pair carefully you will see that there is small shift between the objects in the foreground (like the tip of the baseball bat) vs. the background.

In a previous tutorial we learned that the two members of a stereo pair are not identical but they have small differences, known as stereoscopic deviations. In a properly recorded and aligned stereo pair these deviations are of small magnitude and along the horizontal direction only. Upon fusing these pictures, our brain converts the deviations to depth.

	WRONG	WRONG		RIGHT	RIGHT
2

Discussion:

This is perhaps the trickiest to identify. At first sight it appears that a mounting error is involved. If you place a horizontal ruler over the picture you will notice that the background is out of alignment (left picture higher than right). However, the foreground is in good alignment. So, how can that happen?

The answer is shown in the right. This is how this stereo pair was recorded. It is a true stereo pair with stereoscopic deviations in the horizontal direction. By rotating the pictures by 90 degrees, we have brought the deviations in the vertical direction. Now this becomes an alignment issue (in a properly-aligned stereo pair there should be no deviation in the vertical direction). The foreground (by the stereo window) appears to be aligned because the deviation is zero, but as we move away the deviation increases, hence the misalignment becomes worse.

Two morals from this story: 1) Turning the camera 90 degrees results in a stereo pair that looks unnatural. 2) Trying to correct by rotating the chips by 90 degrees will not work because the stereoscopic deviations will appear in the vertical direction, making it impossible to fuse & enjoy the stereo pair.

	WRONG	WRONG		RIGHT	RIGHT
3

Discussion:

I took this picture with my digital camera and a slide bar and I am showing it here uncropped.

Some people cannot find anything wrong with this stereo pair. But, consider our discussion on the stereo window and the “Three L Rule”: Left eye sees Less in the Left side. Here the left eye clearly sees MORE in the left side. So the stereo window is off.

This is the result of taking the pair by shifting the lens parallel to itself (with the help of the slide bar). The window is at infinity and becomes painfully obvious (at least for me!) in a close up.

To place a better stereo window you must crop the left side of the left picture and the right side of the right picture (in equal amounts). The result is shown in the right.

NOTE: These problems could be identified WITHOUT viewing the pairs in stereo.
However, viewing them in stereo makes it easier to see what is wrong with them.

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This Quiz was brought to you by the Ohio Stereo Photographic Society