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Dynamic Rigs
Though the simplest rigs bolt the two cameras together at a fixed location, more sophisticated rigs allow the cameras to move during a shot.
The simplest and most useful motion may not be what you think: it is to change the inter-ocular distance on the fly. This preserves the proper 3-D sensation, while avoiding extreme vergence angles that make it difficult to keep everything on-screen in the movie theater.
The more complex effect is to change the vergence angle on the fly. This must be done with extreme caution: unless the rig is very carefully built, changing the vergence angle may also change the inter-ocular distance—or even change the direction between them as well. If a rig is to change the vergence angle, it must be constructed to locate the camera nodal point exactly at the center of the vergence angle’s rotation.
A rig that changes only the inter-ocular distance does not have to be calibrated as carefully. A changing IOD should always be exactly parallel to the line between the camera nodal points, which in turn means that on a one-toe camera, the non-moving camera must be perpendicular to the translation axis, or a two-toe camera must have equal toe-in angles relative to the translation axis.
The penalty for a rig that does not maintain a well-defined relationship between the cameras is simple: it must be treated as two separate cameras. The most dangerous shots and rigs are those with changing vergence, either with mirrors or directly, where the center of rotation does not exactly match the nodal point. Unless you have calibrated, it will be wrong. You will be in the same boat as people who shoot green-screen with no tracking markers—and that boat has a hole…
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