the Horopter

The horopter is an imaginary curved surface in space where objects are seen with both eyes focusing on them, so their images fall on the corresponding points of each retina. When an object lies on the horopter, it appears single and stable because your brain receives matching images from both eyes. Objects outside the horopter cause double images (double vision), as the images fall on non-corresponding points. The shape of the horopter varies depending on distance and eye positioning, but it essentially defines the zone where depth perception is most accurate through binocular vision.