Left and right cameras—or dual lens elements—with precise inter-axial spacing. Convergence and interocular distance demand millimeter accuracy.
Two cameras or lens elements at an exact distance from each other – this is the basic requirement for stereoscopic work. The distance between the optical centers is called the Interocular Distance (IOD), and it must be accurate to the millimeter. Too close together and the spatial effect collapses; too far apart and the viewer gets a headache in the cinema. For feature films, we typically orient ourselves to the human interpupillary distance – about 65 millimeters – but can vary depending on the subject and depth effect. For large-format shots or extreme depth of field, we deliberately go beyond the natural interpupillary distance to enhance the stereoscopic effect.
On set, this means: Either you work with two synchronized cameras mounted on a special rig, or you use a beam splitter that equips a single camera with two lenses. Both systems require convergence – the adjustment of the optical axes towards each other. If the cameras run parallel side-by-side, problems arise during editing. We must angle the axes so that the image planes meet at the subject – precisely where the maximum spatial depth is intended to occur. This is not trivial: a millimeter of error over ten meters of subject distance can already lead to double images or eye strain.
The baseline – the physical distance between the lenses – also determines how strong the parallax is. A larger baseline creates stronger depth effects but also presents greater convergence problems. For close-ups, we often significantly reduce the IOD, otherwise the depth becomes exaggerated and unbelievable. With extremely wide-angle lenses, we can even return to a parallel arrangement – the depth effect is created by the perspective anyway.
In the practical workflow, eye mismatch correction in the DI is essential: small convergence errors or height differences between the cameras can be corrected digitally afterward, but they cost time and sharpness. Precise adjustment on set is better. Modern stereoscopic rigs have motorized convergence adjustment – this saves frustration and allows for smooth operation without constant readjustment. Anyone shooting in 3D should also recalculate the interaxial setting depending on the lens focal length to maintain consistent spatial effects throughout the entire film.