Plate camera

The earliest cameras produced in significant numbers used sensitised glass plates and are now termed plate cameras. Light entered a lens mounted on a lens board which was separated from the plate by an extendible bellows. Many of these cameras, had controls to raise or lower the lens and to tilt it forwards or backwards to control perspective. Focussing of these plate cameras was by the use of a ground glass screen at the point of focus. Because lens design only allowed rather small aperture lenses, the image on the ground glass screen was faint and most photographers had a dark cloth to cover their heads to allow focussing and composition to be carried out more easily. When focus and composition were satisfactory, the ground glass screen was removed and a sensitised plate put in its place protected by a dark slide. To make the exposure, the dark slide was carefully slid out and the shutter opened and then closed and the dark slide replaced. Glass plates were later replaced by sheet film in a dark slide for sheet film; adaptor sleeves were made to allow sheet film to be used in plate holders. In addition to the ground glass, a simple optical viewfinder was often fitted.[20] Cameras which take single exposures on sheet film and are functionally identical to plate cameras are still used for static, high-image-quality work; see Large-format camera, below. Photographic plates preceded photographic film as a target medium in photography. A light-sensitive emulsion of silver salts was applied to a glass plate. This form of photographic material largely faded from the consumer market in the early years of the 20th century, as more convenient and less fragile films were introduced. However, photographic plates were still in use by some photography businesses until the 1970s,[1] and were in wide use by the professional astronomical community as late as the 1990s. Such plates respond to ~2% of light received. Glass plates were far superior to film for research quality imaging because they were extremely stable and less likely to bend or distort, especially in large-format frames for wide-field imaging. Early plates used the very inconvenient wet collodion process which was replaced late in the 19th century by gelatin dry plates. In photography, a sheet of ground glass is used for the manual focusing in some still and motion picture cameras , the ground-glass viewer is inserted in the back of the camera, and the lens opened to its widest aperture. This projects the scene on the ground glass upside down. The photographer focuses and composes using this projected image, sometimes with the aid of a magnifying glass (or loupe). In order to see the image better, a dark cloth is used to block out light, whence came the image of the old-time photographer with his head stuck under a large black cloth. A ground glass is also used in the reflex finder of an SLR or TLR camera. In motion picture cameras, the ground glass is a small, usually removable piece of transparent glass that sits between the rotary disc shutter and the viewfinder. The ground glass usually contains precise markings to show the camera operator the boundaries of the frame or the center reticle, or any other important information. Because the ground glass is positioned between the mirror shutter and the viewfinder, it does not interfere with the image reaching the film and is therefore not recorded over the final image, but rather serves as a reference for the camera operator. Ground glasses commonly serve as a framing reference for a desired aspect ratio. Because most films shot with spherical lenses are shot full-frame and later masked during projection to a more widescreen aspect ratio, it is important not only for the operator to be able to see the boundaries of that aspect ratio, but also for the ground glass to be properly aligned in the camera so that the markings are an exact representation of the boundaries of the image recorded on film.