The focal length can be thought of how long a lens is. Long lenses, like a photographer uses at a football game, makes things appear close up. Short lenses make things look further away but give you a wide angle of view. Now, let’s get a little more technical.
The focal length of a lens is the distance between the optical center of the lens and the points where a clear image is formed. We measured the focal length of several magnifying glasses which are one piece, or “simple,” lenses. A camera lens is usually composed of several individual lenses and is called a compound lens. The focal length of modern camera lenses and telescopes is usually measured in millimeters.
Short focal length lenses gives you a wide view and are called wide angle lenses. Longer focal length lenses have a narrow view and make things appear closer. They are called telephoto lenses. In between are normal lenses which have an angle of view similar to the human eye. 35mm cameras use film that is 35mm wide.
Common wide angle focal lengths for 35mm cameras are 24mm, 28mm, and 35mm.
Common normal focal lengths for 35mm cameras are 50mm and 55mm.
Common telephoto lenses for 35mm cameras are 100mm, 135mm, 200mm, 300mm and 400mm.
Zoom lenses are popular today. A zoom lens has a range of focal lengths. The photographer changes the focal length with a button on the camera or a ring on the lens. Examples of common zoom lenses for 35mm cameras are 28mm to 80mm, 70mm to 210mm and 100mm to 300mm.