From cameraobscuras.com:
Although the camera obscura began as a simple pinhole in one all of a darkened room, the arrangement diagramed here produces a brighter, sharper and erect image of great utility. Light from the scene enters the instrument, often through a glass window, and is then reflected vertically downward by a first-surface mirror.
Directly below the mirror is a lens, which focuses the light onto a matte white table.
Observers around the table see a full color image of the outside world; those viewers with their back to the scene see an image that is right side up. Most instruments have controls to rotate and tilt the flat mirror so as to change the area of land or sky that is presented on the table. More advanced instruments use motors to vary the focus and field of view.
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