
The San Francisco Camera Obscura projects an image onto a horizontal viewing table via a reflected image from a viewpoint at the top of the building. A metal hood in the cupola at the top of the building slowly rotates, making a full revolution in about six minutes, allowing for a 360º view around the building. Light enters the building via an angled mirror in the metal hood. It then passed through a lens with a 150 in. (381 cm) focal length and is projected onto a parabolic white "table" in a black room. When the fourth Cliff House opened in 1937, the owner was approached by businessman Floyd Jennings with the idea of adding a camera obscura to the cliffs behind the restaurant. It was installed on the site in 1946 and has been in continuous operation since then. The Camera Obscura was added to the National Register of Historic Places in 2001 for its engineering significance. While the exterior of the building was extensively modified in 1957 to appear as a giant camera, the internal workings of the camera obscura, the basis of its placement on the Register, remained unchanged.
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