base separation rules for stereo capturing


From John’s 3D Guide
http://www.crystalcanyons.net/pages/3dguidebook/BasicMethods.htm

HYPO AND HYPERSTEREO

From this we learn that the stereo base is one element in defining the perceived depth. Another factor is image magnification. In hypostereo the stereo base is less than the normal eye separation of 65mm (or about 2.4 inches). In hyperstereo the stereo base is more than normal eye separation. In ortho-stereoscopic viewing the base separation and the focal lengths of the lenses (43mm = eye equivalent) are set up to reproduce what the viewer would see in reality. In an “orthostereo display”, the taking and viewing lenses are usually the same. The two 35mm cameras mounted as shown at the bottom of figure 1.1 have a typical separation of about 3 inches. This is slightly hyperstereoscopic and may be just noticeable in normal viewing. With a large hyperstereo base distance a person will appear taller or more leggy than in actual fact. Sometimes hyperstereo is a good thing because it exaggerates the stereo effect! On the other hand it can render near objects oddly. For most scenes and action shots where the subject is not too close, the slightly hyper configuration at the bottom of figure 1.1 is fine.

Hyperstereo (extended stereo base) is particularly useful in shots where the nearest object is far off. Human’s do not notice depth on objects more than a couple hundred feet away because the rays coming to the eyes from such a distant object are essentially parallel. However, by moving the “eyes” (i.e. the cameras) further apart, such rays now come in at differing angles and depth perception is restored. For example, if the nearest object is 100 feet away, a hyperstereo base with the cameras separated by 1 to 2 feet will render significant depth to the image. Cameras three feet apart will yield stereo pairs that show some depth even at subject distances of 1000 feet or more.

IMPORTANT: When viewing objects as illustrated in figure 1.3, if the fig tree separation between L and R increases without limit, the left and right eyes will be looking at such disparate scenes that the brain won’t be able to “fuse” the image into a rational scene. Trying to fuse an image with too much near vs. far separation produces severe strain, nausea, even migraine headaches. Thus there is a fundamental guideline of 3D photography:

THE BASE SEPARATION FOR A NORMAL LENS (i.e. 50mm on a 35mm camera) SHOULD NOT EXCEED 1/30 THE DISTANCE TO THE NEAREST OBJECT IN THE FIELD OF VIEW.

Examples:

a) Nearest object 20 feet. Maximum separation 20/30 feet ~ 8 inches.

b) Nearest object 100 feet. Maximum separation 100/30 ~ 3 feet.

c) Nearest object 20 feet but a wide angle 24mm lens is used. Since the wide angle produces an image where the angular displacements are about half what they are with the 50mm lens (i.e. it is “twice as wide”), the maximum separation is 2*20/30 ~ 1.3 feet. Alternatively for a 24mm lens the guideline is 1/15.

d) Nearest object 100 feet, but we want to use a 135mm telephoto to “bring the object closer”. In doing this, image displacements are magnified by 135/50 = 2.7. Thus the maximum separation is about 20/(2.7*30) feet or approximately 1/4 foot ~ 3 inches.

e) You don’t always want to push the limit on the 1/30 rule. Experimentation will show the way. Telephoto lenses seem to magnify all the potential errors (parallelism in pointing for example), while wide angles generally are more forgiving and better for 3D because they don’t compress distance and do not exaggerate pointing errors.

NOTES ON THE 1/30 RULE:

Some experienced 3D’ers consider The 1/30 Rule to be more of a myth than a rule. In the Art of photography, there are few canonical rules. 1/30 is a good guideline to start with, however. Exceptions abound in reality. For example, if the near point and the far point in the scene are close in distance (for example if the far point is not at infinity, but only 10 feet in back of a 10 foot near point), then 1/30 can sometimes be exceeded (maybe to something like 1/15). In macro work the near points and far points are usually very close (because there is little depth of field when the magnification is high). Nonetheless I prefer not to exceed 1/30 in such situations. The ability to fuse stereo images varies widely from person to person. This ability also varies with the method of presentation (viewer magnification, slide projection beam angle (or projection lens magnification), distance from the screen, etc.) If you are going to expose a wide range of folks to your 3D art, it may be best not to go wild with the separation. Two migraines in a large audience are still two to many. Experimentation is a good way, at least initially. Bracket (i.e. try the same scene) with a few different stereo bases and test it out (writing down what you did!).

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