Hold a finger up and blink one eye, then the other — it jumps. That jump, the DISPARITY between what your two eyes see, is bigger for near things and tiny for far ones, and your brain reads it straight off as depth. Two eyes are a triangulation rig you were born with. Slide the object closer and watch the disparity grow.
Binocular stereopsis is triangulation with a fixed baseline: the two eyes (separation B ≈ 65 mm, focal length f) see a point at slightly different image positions, and that DISPARITY d relates to depth by Z = f·B/d. Near objects give large disparity, far objects vanishingly small — which is why stereo depth fades with distance. The brain fuses the two retinal images and reads Z off the disparity map (random-dot stereograms prove it needs nothing but disparity). A fail-loud self-check throws unless larger disparity means nearer. ◆ real visual neuroscience / photogrammetry, node-verified.
The pinhole Z=fB/d relation (exact for aligned cameras/eyes); real stereo needs the correspondence solved first (which point matches which) and vergence — the disparity-is-inverse-depth law is the exact core, shared with every stereo camera.