Cameras may work with the light of the visible spectrum or with other portions of the electromagnetic spectrum. A camera generally consists of an enclosed hollow with an opening (aperture) at one end for light to enter, and a recording or viewing surface for capturing the light at the other end. A majority of cameras have a lens positioned in front of the camera's opening to gather the incoming light and focus all or part of the image on the recording surface. The diameter of the aperture is often controlled by a diaphragm mechanism, but some cameras have a fixed-size aperture.
Most cameras use an electronic image sensor to store photographs on flash memory. Other cameras, particularly the majority of cameras from the 20th century, use photographic film.The actual name of camera obscura was applied by mathematician and astronomer Johannes Kepler in his Ad Vitellionem paralipomena of 1604. He later added a lens and made the apparatus transportable, in the form of a tent. British scientist Robert Boyle and his assistant Robert Hooke developed a portable camera obscura in the 1660s. The first camera obscura that was small enough for practical use as a portable drawing aid was built by Johann Zahn in 1685. At that time there was no way to preserve the images produced by such cameras ex
ept by manually tracing them. However, it had long been known that various substances were bleached or darkened or otherwise changed by exposure to light. Seeing the magical miniature pictures that light temporarily "painted" on the screen of a small camera obscura inspired several experimenters to search for some way of automatically making highly detailed permanent copies of them by means of some such substance.
Due to the optical properties of photographic lenses, only objects within a limited range of distances from the camera will be reproduced clearly. The process of adjusting this range is known as changing the camera's focus. There are various ways of focusing a camera accurately. The simplest cameras have fixed focus and use a small aperture and wide-angle lens to ensure that everything within a certain range of distance from the lens, usually around 3 metres (10 ft) to infinity, is in reasonable focus. Fixed focus cameras are usually inexpensive types, such as single-use cameras. The camera can also have a limited focusing range or scale-focus that is indicated on the camera body. The user will guess or calculate the distance to the subject and adjust the focus accordingly. On some cameras this is indicated by symbols (head-and-shoulders; two people standing upright; one tree; mountains).