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The horizon of a scene is the edge of vision at the height of the camera, parallel with the world coordinate plane. You can view the horizon in camera viewports.

Horizon in the distance of a scene

A camera is level when it and its target are the same height from the world coordinate plane. In other words, the camera's local Z axis is parallel to the world plane. When the camera is level, the horizon line is centered in the viewport. As the camera tilts up, the horizon line lowers; as it tilts down, the horizon line raises.

The horizon line can help you match the perspective of your scene to the perspective of a still image. In general, matching perspective involves the following steps:

If you raise or lower the camera, raise or lower the target by an equal amount, in order to keep them level and maintain the horizon. See Using the Horizon to Match Perspective.