
Originally Posted by
jfrancis
On a flat, infinite plane, a LEVEL camera will always see a horizon dead in the center. No matter how far above the camera rises above that infinite plane, it will ALWAYS see a horizon line dead in the center, because the plane itself is infinite. (there are no infinite planes in the real world, but our planet seems like one for most practical purposes)
This is a little counter-intuitive, so it bears repeating: on an infinite plane, a LEVEL camera will ALWAYS see a horizon line dead in the center NO MATTER HOW FAR ABOVE the infinite plane it rises.
The ONLY way to raise or lower the horizon line in a normal, uncropped image is not to raise the camera (Y-translation) but to TILT the camera (camera X-rotation) so that it is no longer LEVEL but instead it looks a bit up or a bit down.
A high horizon ONLY** happens when a camera is tilted so as to look somewhat downward. Therefore a high horizon always goes hand-in-hand with keystoning verticals that converge toward the bottom of the image and spread toward the top, like you are looking down into the grand canyon.
A low horizon ONLY** happens when a camera is tilted so as to look somewhat upward. Therefore a low horizon always goes hand-in-hand with keystoning verticals that converge as they rise, like you are looking up at tall buildings.
**You can take one of the above scenarios and CROP it so as to try and fool someone into thinking it is the other scenario, but that would be a confusing and not particularly illuminating or helpful thing to do -- with one exception...
...It is generally considered not only acceptable, but creatively desireable to take a LEVEL CAMERA (horizon dead center, verticals pretty close to vertical) scenario and RECROP it so as to favor sky over earth (or vice versa). This happens all the time because we generally consider a horizon line which cuts a picture in half to be a poor composition.
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