When we speak of a camera or camera obscura it means the image forming device and photographic film (or nowadays a digital storage card) is the recording medium.
Other methods are also available. For example, the photocopy or xerography machine forms permanent images but uses the transfer of static electrical charges rather than photographic film, hence the term electrophotography. The rayographs published by Man Ray in 1922 are images produced by the shadows of objects cast on the photographic paper, without the use of a camera. And one can place objects directly on the glass of a scanner to produce pictures electronically.
Photographers control the camera to expose the light recording material (usually film or a charge-coupled device) to light. After processing, this produces an image whose contents are acceptably sharp, bright and composed to achieve the objective of taking the photograph.
The controls include:
Focus
Aperture of the lens
Duration of exposure (or shutter speed)
Focal length of the lens (telephoto, macro, wide angle, or zoom)
Sensitivity of the medium to light intensity and color/wavelength
Filters, scrims, or other special effects that may be placed between the subject and the light recording material, either in front of or behind the lens
The nature of the light recording material itself, for example, its resolution as measured in pixels or "grains" of silver halide
The controls are inter-related, as the total amount of light reaching the film plane (the "exposure") changes proportionately with the duration of exposure, aperture of the lens, and focal length of the lens (which changes as the lens is focused, or if it is "zoomed"). Changing any of these will, therefore, alter the exposure. Many lenses will automatically adjust the aperture to account for changes in focus, and some will do so for changes in zoom as well.
The duration of an exposure is referred to as the "shutter speed," often even in cameras that don't have a physical shutter, and is typically measured in fractions of a second. The aperture is expressed by an f-number or f-stop, which is proportional to the ratio of the focal length to the diameter of the aperture. If the f-number is decreased by a factor of , the aperture diameter is increased by the same factor, and its area is increased by a factor of 2. The f-stops that might be found on a typical lens include 2.8, 4, 5.6, 8, 11, 16, 22, 32, where going up "one stop" doubles the amount of light reaching the film, and "stopping down" one stop halves the amount of light.
In order to achieve a given exposure, various combinations of shutter speed and aperture could be used. For example, f/8 at 1/60th of a second and f/4 at 1/120th of a second yield the same amount of light. However, the combination chosen impacts the final result. In addition to the subject or camera movement that might vary depending on the shutter speed, the aperture (and focal length of the lens) determine the "depth of field," which refers to the range of distances from the lens that will be considered in acceptable focus. For example, using a long lens and a large aperture, such as might be used with a large format camera, a subject's eyes might be in sharp focus while the tip of his nose is noticeably blurred. If the aperture is made smaller, or a shorter lens is used, then both the subject's eyes and nose can be brought into focus at the same time. If a very small aperture is used, such as a pinhole, then a very wide range of distance can be brought into focus at once.
Finally, image capture is only half of the image forming process. Regardless of the light recording material used, some sort of process or processes must be employed to render the "latent image" captured by the camera into the final photographic work. In addition to the camera controls, the processing variables have a significant impact on the final result.
