focal length 
A camera lens refracts light in such a way all the varying wavelengths converge, or focus, at the film (in a traditional camera) or sensor plane (in a digital camera). The distance between the lens and the film or sensor plane is called the focal length. It's typically measured in millimeters and is indicated on the lens barrel. The longer the focal length, the greater the camera can magnify a distant object. To maximize the amount of light of all wavelengths reaching the sensor, a digital camera's lens is much closer to the sensor plane than the film is in a traditional 35mm camera, so its focal lengths are accordingly smaller. Because the area of the scene covered at a given focal length (known as the angle of view) changes with the size of the film or sensor plane, digital camera vendors often list the focal length of a traditional 35mm camera that would produce the equivalent angle of view. Because digital single-lens-reflex cameras (dSLRs) can take multiple lenses, the vendors will instead provide a focal length multiplier so that users can calculate the equivalent angle of view for any lens.