Consistently, the shape of the empirical horopter have been found to deviate from the geometrical horopter. For the horizontal horopter this is called the Hering-Hillebrand deviation. The empirical horopter is flatter than predicted from geometry at short fixation distances and becomes convex for farther fixation distances. Moreover the vertical horopter have been consistently found to have a backward tilt of about 2 degrees relative to its predicted orientation (perpendicular to the fixation plane). The theory underlying these deviations is that the binocular visual system is adapted to the irregularities that can be encountered in natural environments.