The focal length of a lens determines one and only one thing: the angle of view encompassed by the frame of film or digital sensor. Restricting this discussion to 35 mm cameras, a 50 mm focal length lens views a 47° wide patch of the scene (measured on the diagonal of the frame). This results in a view similar to what the human eye sees, and the lens is called a normal lens. Focal lengths shorter than 50 mm give wider views and are called wide angle lenses. A 28 mm lens sees a 76° field of view, for example. Lenses with focal lengths longer than 50 mm are called long lenses (or commonly but mistakenly telephoto lenses). Long lenses view narrower slices of the scene than normal lenses; a 200 mm lens views a tiny 12° angle out of all that's in front of the camera.
Wide angle images.
Long lenses work just the opposite. They compress the sense of
space in an image. The narrow angle of view forces you to be far away from
what you are photographing, so that everything in the frame is relatively
far away and can appear to be almost at the same distance from the camera.
Those traffic report cameras, exaggeratedly showing freeway traffic almost
touching bumpers, use long lenses.
Long focal length images.
Although lens focal length strictly speaking only determines the angle of view of the lens, this angle of view forces the photographer into certain ways of shooting. The upshot is that the best way to think about lens focal length is that
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