Difference between left and right camera images that the brain interprets as depth — foundation of stereoscopy. Interocular distance and convergence angle control the effect.
Your left and right eyes see the world from slightly different positions. The brain uses precisely this offset—retinal disparity—to calculate depth. On set, it works the same way: two cameras, spaced at the distance of human interpupillary distance (approx. 65 mm, the interpupillary distance), capture the same scene from minimally offset viewpoints. The viewer, using polarized or shutter glasses, then sees two images that their visual system merges into a spatial perception.
In practice, timing is crucial. Your convergence angle—the angle at which the two camera views meet—determines where the "zero plane" lies, the point where disparity is zero and the viewer feels no depth effect. If you set the cameras too steeply towards each other (too narrow a convergence angle), "window violations" occur: objects unnaturally cut through the frame. Too shallow an angle, on the other hand, makes the 3D effect appear flat. Professionals work with toe-in or horizontal image shift—two different techniques to control this angle without twisting the optical axis.
Disparity itself is not constant. The greater the object distance, the smaller the disparity between the left and right image; the closer an object, the larger it is. This is biologically correct—your eye clearly perceives this during close-ups in 3D cinema. Your eye muscles tense as if they were actually converging. This is why stereoscopy only works if the disparity remains within a certain range—values that are too extreme lead to eye fatigue and headaches.
On set, this means: calculate your baseline (camera distance) according to the scene's space. Tight spaces require smaller baselines (30–40 mm) to avoid over-convergence. Landscape shots can handle 65 mm or more. And don't forget: lens choice massively changes perceived disparity. A wide-angle lens creates stronger depth effects than a telephoto lens, even though the camera distance remains the same—because the perspective itself is different. This is not a mistake, but a creative choice.