In stereoscopic 3D, the angular inward angle of both camera lenses — misalignment causes eye strain and headaches. Calibrated on-set or in post.
Convergence determines how the two camera lenses are aligned with each other in stereoscopy. While a single optical axis exists in normal 2D shooting, 3D works with two parallel or converging rays – and this is precisely where the calculation begins. Convergence is the angle at which these two lenses would "meet" if extended forward. Ideally, they converge exactly on the plane the viewer is intended to focus on – the depth staging then appears natural without causing discomfort.
In practice, convergence was long the cornerstone of stereoscopic shooting. You set up your two cameras parallel to each other – perfect for viewer comfort, but not converging. As soon as objects come closer than about two to three meters, disparity arises: each eye sees something different, and the brain can no longer fuse them without difficulty. This leads to eye strain, headaches, discomfort – everything that ruined 3D films. The old method was therefore to mechanically converge the cameras, i.e., to tilt their optical axes inward. This works, but costs image quality and control in editing.
Today, convergence is primarily calibrated in post – that's the clean way. In editing – or more precisely, in stereoscopic finishing – software is used to shift the convergence plane while controlling horizontal disparity. Some systems even allow dynamic real-time convergence adjustment within a shot. However, this requires precise depth maps and exact camera calibration on set – focal length, sensor size, and interaxial distance must be documented like a bible. Missing or insufficient data in the original means guesswork and compromises later, which can damage the image.
The critical lesson: convergence is not a cosmetic problem that can be ignored. It is the foundation for stereoscopy to work at all. On set, you need a team that understands where the convergence plane needs to be – at what depth. This is determined with convergence markers, test shots, and monitor setup. In editing, it's your job as a stereo supervisor or post-supervisor to ensure that the convergence doesn't jump wildly (which guarantees headaches) and that it harmonizes with the film's narrative depth information. Don't underestimate this – bad convergence is one of the main reasons why 3D fails.