Mirror rig
In a side-by-side rig, the interaxial of the two cameras cannot be smaller than the width of their bodies and lenses. A mirror rig (beam-splitter rig) removes this limit: the interaxial can start from zero.
When it is needed
A small interaxial is needed for close objects, for wide-angle lenses and for shots made for large screens. For example, on a 10 m screen the largest interaxial without divergence, with a 35 mm lens and 3.5 m convergence, is 16 mm (see Interaxial). No cinema camera can be set up side by side at this distance.
Construction
A semi-transparent mirror is placed at 45° to the optical axis. One camera looks from behind the mirror, through the light that passes through it. The other camera stands perpendicular to the mirror and looks through the light the mirror reflects. The virtual position of the reflected camera in the mirror falls next to the transmitted camera; the horizontal distance between them is the interaxial, and it can be reduced to zero.
Costs
- Mirrored image: the image of the reflected camera is a mirror image; it is flipped in post-production.
- Light loss: the light is split between the two cameras. With a mirror that splits it evenly, each camera receives about half the light, that is, one stop less.
- Difference between the two eyes: the mirror affects transmitted and reflected light differently depending on polarization. In bright reflections, water and sky, differences in brightness and color can appear between the two images; they are matched in mastering.
- Sensitivity: the mirror is sensitive to dust, vibration and heat; the rig is large and heavy, and its calibration takes time.
For distant landscapes and wide scenes that need a large interaxial, a side-by-side rig is simpler.