Eye level — where it appears
Named by 2 essays across one field — each of them below, with the objects they name alongside it.
The horizon is at eye level — if the picture plane is vertical
The horizon cuts every standing figure at the same fraction of its height however far away it is, which is the most immediately usable fact in the subject. It holds when the camera is level, and a twelve-degree tilt is enough to spread the fractions by more than a percentage point.
The horizon, and the fraction
The horizon crosses every upright at the point of it that stands at the camera's own eye height — always, whatever the picture plane is doing. It crosses at the same *fraction* of the drawn height only when the plane is vertical: tilt by 6° and the fractions spread by 0.08 percentage points, tilt by 4° and 0.06. One statement is an incidence and survives; the other is a ratio and does not.
Named alongside it
The objects these essays reach for when they reach for this one.
HorizonPicture planeAffine mapCamera tiltCross ratioForeshorteningnecessary, not sufficientpoint at infinitysingle-view metrologyVanishing lineVanishing pointvertical vanishing point