The camera obscura
(Lat. dark chamber) was an optical device used in drawing, and one of the ancestral threads leading to the invention of photography. Photographic devices today are still known as "cameras".
The principle of the camera obscura can be demonstrated with a rudimentary type, just a box (which may be room-size) with a hole in one side, (see Pinhole cameras for construction details). Light from only one part of a scene will pass through the hole and strike a specific part of the back wall. The projection is made on paper on which an artist can then copy the image. The advantage of this technique is that the perspective is right, thus greatly increasing the realism of the image (correct perspective in drawing can also be achieved by looking through a wire mesh and copying the view onto a canvas with a corresponding grid on it). With this simple do-it-yourself apparatus, the image is always upside-down. By using mirrors, as in the 18th century overhead version illustrated, it is also possible to project a right-side-up image. Another more portable type, as in the second drawing, is a box with an angled mirror projecting onto tracing paper placed on the glass top, the image upright as viewed from the back.
As a pinhole is made smaller, the image gets sharper, but the light-sensitivity decreases. With too small a pinhole the sharpness again becomes worse due to diffraction. Practical cameras obscura use a lens rather than a pinhole because it allows a larger aperture, giving a usable brightness while maintaining focus.
Some cameras obscura have been built as tourist attractions, often taking the form of a large chamber within a high building that can be darkened so that a 'live' panorama of the world outside is projected onto a horizontal surface through a rotating lens. Although few now survive, examples can be found in Grahamstown in South Africa, Bristol and Portslade village in England, Aberystwyth and Portmeirion in Wales, Kirriemuir, Dumfries and Edinburgh in Scotland, Lisbon in Portugal, and Santa Monica and San Francisco in California, Havana in Cuba, Eger in Hungary, and Cádiz in Spain.