The distance from the center of the lens to the image of an object at infinity. At same size, the distance from copy to image is four times the focal length of the lens. This also applies to lithographic process lens as well. A 35mm SLR camera has interchangeable lenses that can range in focal length from around 7 to 17mm which are usually fisheye, 17 to 35 are considered wide angle. 50mm is called the standard lens as it has the same equivalent focal length as the human eye. Anything longer is a telephoto lens. Usually expressed in millimeters, smaller focal lengths result in wider fields-of-view.