The horopter is the set of points that projects at the same location in the two retinae (i.e. that have the same visual direction).
This is the set of points that correspond geometrically to the intersection between visual lines at identical eccentricities.
But the horopter can also be defined as the center of the Panum's fusional area, the apparent fronto-parallel plane or the equal distance from fixation.
[4] At short fixation distances, the empirical horopter is a concave parabola flatter that a circle.
But this is true only for short fixation distances where the empirical horopter is intermediate between these two set of points.
Binocular geometry. The absolute disparity is the angle between visual lines that intersect at a given point. The relative disparity is the difference between the absolute disparity of 2 points. The Vieth–Müller circle, or horizontal geometrical horopter, is the set of points that have 0-relative disparity to fixation (thus the same absolute disparity as fixation). Geometrically this is a circle passing through the nodal point of the 2 eyes and through fixation. The empirical horopter, measured according to a given criterion such as identical visual directions in the 2 eyes, does not fall on the geometrical horopter. It is concave as short fixation distances, flat at the abathic distance and then convex.
