Article by www.nikonusa.com
 
Focal length, usually represented in millimeters (mm), is the basic description of a photographic lens. It is not a measurement of the actual length of a lens, but a calculation of an optical distance from the point where light rays converge to form a sharp image of an object to the digital sensor or 35mm film at the focal plane in the camera.
The focal length tells us the angle of view—how much of the scene will be captured—and the magnification—how lar...