Focal Length  

When parallel rays of light strike a lens focused at infinity, they converge to a point called the focal point. The focal length of the lens is then defined as the distance from the middle of the lens to its focal point. The focal length of a lens is usually displayed on the lens barrel and is measured in millimetres.  

The field of view or the focal length of a lens represents how much of a given scene (the coverage area) a lens can take in. When taking pictures of a group of friends, using a wide-angle lens (small focal length) allows you to fit more friends in to the picture. Using a telephoto lens (large focal length) allows you to pick out one friend from the group in order to take a close-up picture.   
Typically a lens with a focal length of 35mm or less is called a wide-angle lens, while a lens with a focal length of 70mm or more is called a telephoto lens.  



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