Depth plane curvature
A flat wall parallel to the camera does not look flat when shot with a converged rig: its edges recede relative to its center. This is called depth plane curvature (Woods, Docherty, Koch 1993).
Why it occurs
In a converged rig, points at the same distance from the camera do not receive the same parallax everywhere in the frame. At the center of the frame, the point at the convergence distance is at zero parallax; a point at the same distance but at the edge of the frame receives positive parallax. On screen this difference moves the edges of a flat surface away from the viewer.
Magnitude
Curvature increases as the interaxial grows and as the convergence comes closer. On a TV 55" at a viewing distance of 2.5 m, the edge of a flat wall filling the width of the frame is perceived this far behind its center:
| Shot | Wall at the convergence distance | Wall at twice the distance |
|---|---|---|
| interaxial 65 mm, convergence 3.5 m | 16 cm | 30 cm |
| interaxial 150 mm, convergence 2 m | 83 cm | the edges diverge (the eyes have to turn outward) |
Correction
In a parallel rig, and in a converged shot with its keystone corrected, a flat surface parallel to the camera receives constant parallax and is perceived as flat. The keystone correction done in mastering also removes the curvature; when After mastering is selected in the diagram, the walls become flat. The MASTERING / AS SHOT button in the calculator shows the same difference in perceived space.
Sideways movement
Woods et al. mention one more distortion: as the viewer moves sideways from a position facing the screen, the perceived scene leans sideways around the screen plane (shear distortion). Objects in front of the screen appear to move in the same direction as the viewer, objects behind it in the opposite direction. This distortion comes from the viewing position, not from the shot.