Victorian optical device presenting offset images to each eye — ancestor of all modern 3D. Today: umbrella term for any stereoscopic viewing apparatus from polarized cinema glasses to VR headsets.
Two images, each to a different eye — that's the core principle that has been working since the 19th century and remains the basis of all 3D processes to this day. The human brain merges these slightly offset perspectives into spatial depth. On set, you notice this immediately: as soon as you plan for stereoscopy, you're not working with one camera, but with two sensor blocks or two complete cameras whose axes and distances must be precisely calibrated. The interaxial distance — that is, the distance between the two lenses — determines the intensity of the depth effect. Set too wide, it tires the eye. Too narrow, and the 3D illusion flattens out.
In practice, you distinguish between different types of viewing aids: The classic anaglyph glasses with red-cyan filters have long been obsolete because they produce color casts and offer only a flat illusion. Polarized glasses — standard in multiplex cinemas today — use crossed polarization filters and enable true color reproduction. In the high-end consumer sector, active shutter glasses are used, which work with LCD shutters synchronized to the frame rates. VR headsets also utilize the stereoscopic effect, but with two separate displays for each eye instead of a common cinema screen.
For you as a DoP, stereoscopy specifically means: new parameters for focus planning, because the convergence plane — the point on which both eyes are focused — must correspond with the stereoscopic window. Errors here create visual strain for the viewer. The lighting must be identical for both cameras, otherwise flickering and artifacts will occur. Color grading is also more critical: any minimal deviation between the left and right channels becomes visible. You need specialized monitoring solutions — simply two 4K displays side-by-side are not enough. The common DCI-3D formats (DCP-3D) are based on JPEG2000 compression with depth encoding, which is why your raw grading work requires a different pipeline than for 2D mastering.
Today, stereoscopy in mainstream cinema — despite the Avatar boom — is rather on the decline. VR and location-based installations are the growing market. For your considerations: stereoscopy is technically feasible, but it is resource-intensive and not fault-tolerant. Any optical or color discrepancy between the channels will set you back in grading. Therefore, stereo 3D projects work with a significantly higher technical budget and longer post-production cycles than classic 2D cinema.