Linton stereo illusion

We present a new illusion that challenges our understanding of stereo vision. The illusion consists of a larger circle at 50cm, and smaller circle in front of it at 40cm, with constant angular sizes throughout. We move the larger circle forward by 10cm (to 40cm) and then back again (to 50cm). The question is, what distance should we move the smaller circle forward and back to maintain a constant perceived separation in depth between the circles? Constant physical distance (10cm) or constant retinal disparity (6.7cm)? Observers choose constant disparity. We therefore argue the ‘Linton Stereo Illusion’ appears to suggest that perceived stereo depth reflects retinal disparities rather than 3D geometry.