Photographic Images
Photography is the process of making pictures by means of the action of light. Light patterns reflected or emitted from objects are recorded onto a sensitive medium or storage chip through a timed exposure. more...
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The process is done through mechanical, chemical or digital devices known as cameras.
The word comes from the Greek words φως phos ("light"), and γραφις graphis ("stylus", "paintbrush") or γραφη graphê, together meaning "drawing with light" or "representation by means of lines" or "drawing." Traditionally the product of photography has been called a photograph. The term photo is an abbreviation; many people also call them pictures. In digital photography, the term image has begun to replace photograph. (The term image is traditional in geometric optics.)
Photographic image-forming devices
A camera or camera obscura is the image-forming device and photographic film or a digital storage card is the recording medium, although other methods are available. For instance, the photocopy or xerography machine forms permanent images but uses the transfer of static electrical charges rather than photographic film, hence the term electrophotography. Rayographs published by Man Ray and others are images produced by the shadows of objects cast on the photographic paper, without the use of a camera. Objects can also be placed directly on the glass of a scanner to produce digital pictures.
Photographers control the camera and lens to expose the light recording material (usually film or a charge-coupled device; a complementary metal-oxide-semiconductor may also be used) to the required amount of light. After processing, this produces an image.
The controls include:
Focus of lens;
Aperture of the lens (amount of light allowed to pass through the lens);
Focal length and type of lens (telephoto, macro, wide angle, or zoom);
Filters, or scrims placed between the subject and the light recording material, either in front of or behind the lens;
Duration of exposure (or shutter speed);
Sensitivity of the medium to light intensity and color/wavelength;
The nature of the light recording material, for example its resolution as measured in pixels or grains of silver halide;
Camera 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 zoomed). Changing any of these controls alter the exposure. Many cameras automatically adjust the aperture of the lens to account for changes in focus, and some will accommodate changes in zoom as well.
The duration of an exposure is referred to as shutter speed, often even in cameras that don't have a physical shutter, and is typically measured in fractions of a second. Aperture is expressed by an f-number or f-stop (derived from focal ratio), 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 
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