Converged rig

In a converged rig (toe-in), the cameras are rotated toward each other by equal angles. Their central rays cross at the convergence distance; an object at that distance is at the center of both images.

Geometry

For interaxial b and convergence distance C, the toe-in angle of each camera is:

where: β the toe-in angle of each camera; the angle between the two cameras is 2β

Convergence is set on location, as seen in the viewfinder, and because the images are not shifted, no cropping at the edges is needed. In return, the two image planes are rotated by 2β relative to each other, and two distortions appear (Woods, Docherty, Koch 1993).

Keystone distortion

On two rotated image planes, a rectangular frame appears as two opposite trapezoids. In the corners of the frame, one eye's image sits above or below the other: vertical parallax. A commonly cited acceptance limit for vertical parallax is about 15 arcminutes.

Example: 35 mm lens, Super 35 sensor. With a 150 mm interaxial and 2 m convergence, the toe-in angle is 2.15°; the corner vertical parallax is 20 pixels in a 1920-pixel image and 17 arcminutes on a 55" TV viewed from 2.5 m. With a 65 mm interaxial and 3.5 m convergence, the corner vertical parallax is 5 pixels.

Depth plane curvature

Points at the same distance from the camera get zero parallax at the center of the frame but positive parallax at the edges and corners. A flat surface is perceived as if its edges were bent backward. In the 150 mm and 2 m example above, the corner of the convergence plane gets a parallax of +1.33% of the image width.

Correction in mastering

Each image is corrected by rotating it back by β about its own center (rectification). The corrected pair has the same geometry as a pair shot parallel with shift, with the same interaxial and convergence. Keystone distortion and curvature disappear; some cropping is needed at the edges of the rotated image.

In the Cerit Triangle the cameras are always converged. The MASTERING / AS SHOT button in the calculator's perceived space view shows the same shot corrected and as shot.