This is a manual for a Camera Lens:

Also known as: Photographic lens, Objective Lens, Photographic Objective. Definition: A camera lens is a combination of curved pieces of glass and is used to focus images on a photographic film or a camera’s sensor. A camera lens has two important specifications – the lens opening and focal length. The lens opening (also known as f-stop) determines the amount of light that enters the lens. This aspect is controlled by the camera’s shutter speed and aperture. The focal length determines the amount of magnification and the angle viewed by the camera. For example, a wide-angle camera lens has a larger focal length, compared to a normal lens. This enables the former to increase the viewing area. Objects in an image taken by a wide-angle camera appear smaller than what they would appear in images taken by a normal camera lens. Focal length is expressed in millimeters (mm); usually, a normal lens used in a digital camera has a focal length of 32mm. In an SLR camera, the lens may be changed according to the image attributes required by the photographer.