A camera is an instrument for recording images on photographic film . It consists of a light – tight box with a shutter , a lens containing a diaphragm and a viewing system . When the shutter is released , the film is exposed to light from the subject that being photographed . Adjusting the shutter speed alters the time for which the film is  exposed to light . The diaphragm , by altering the aperture of the lens controls the interesting of light entering the camera . The total amount of light entering the camera called exposure . The lens focuses the light on to  the film . When there is insufficient light to produce an adequate image , a flash gun may be used to give extra light .