Horizon Line & Eye Level Calculator
Tilt the camera and the horizon moves. Put the horizon where you want it and read off the tilt. Then find where the ground at ten metres lands.
Horizon, eye level and tilt
The horizon line is the eye level of the viewer, and with a level camera it passes through the centre of the picture. Tilting the camera slides it: down brings it up the frame, up pushes it down, by f · tan(tilt) where f is the focal length measured in frame heights. Longer lenses move it faster.
This calculator works in both directions. If you know how the camera is tilted, it tells you where the horizon lands. If you are composing and know where you want the horizon (say 35% from the top), it tells you the tilt that puts it there, which is the number to type into the 3-point grid.
Placing things on the ground
The second half of the calculator answers "where does the ground at ten metres appear?" A point on the ground straight ahead at distance d sits at an angle atan(eye height ⁄ d) below the horizontal. Add the tilt, project through the lens, and you have its position in the frame. That is where a figure standing there puts its feet, where a car's tyres touch, where the base of a lamp post goes. It also tells you how close the nearest visible ground is, which is useful for deciding whether the foreground will be empty.
Related
The figure height calculator does the same for a level camera and adds the figure's size. The vanishing point finder measures the horizon and tilt from a real photo.
Questions
Where should the horizon line be?
At the viewer's eye level. In a drawing that is a choice: low (near the bottom) makes things loom and feels like a child's or a crouching view; high feels like standing on a balcony. For a level camera it sits at the vertical centre of the frame; tilting the camera moves it.
How does camera tilt move the horizon?
Tilt the camera down and the horizon rises in the frame; tilt it up and the horizon drops. The amount depends on the lens: with a long lens a small tilt moves it a lot. The relationship is horizon offset = f · tan(tilt), where f is the focal length in frame units.
Can the horizon be outside the picture?
Yes. Look down steeply into a street or up at a tower and the horizon (and both horizontal vanishing points) sit outside the frame. Vertical lines then converge toward a third point, which is three-point perspective.