Photo Tip: What Lens Focal
Length
Really Means
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.
This seemingly innocuous statement "the focal length determines
only the angle of view" has profound ramifications, however.
Consider using a wide angle lens on your camera. It takes in such a wide
swath of the world, things are likely to be small and jumbled in the image
if you are not careful. It's hard to create a unified composition with a
clear center of interest when everything's a visual jumble. The wide angle
lens, therefore, forces you to move in close to one or a few objects in
the foreground to make an interesting composition. Nature shots with distant
mountains and flowers in the foreground are likely taken with a wide angle
lens. What you wind up with is an image with something very near to the
camera and other things much farther away, so the sense of depth in the
photo is enhanced. The effect of wide angle lenses is to expand the
sense of space.
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
wide angle lenses expand space and long lenses compress space.
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