focal length

A camera lens’ focal length is the distance (usually given in millimeters) from the first lens element to the focal point. The focal length is a description of how aggressively a lens system converges light—the longer a focal length, the more acute the angle of the refracted light, while shorter lengths necessarily refract more sharply to reach focus within the smaller distance. Focal length determines the resulting field of view, with longer focal lengths providing more magnification than shorter lengths.