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The degree of magnification in a camera lens. If it is adjustable, then the lens can Zoom. Non-zoom lenses are called prime lenses. For 35mm film, a wide-angle lens will have a focal length of about 15-35mm, a normal lens about 40-60mm, and a telephoto or long-focus lens about 100-1000mm (or very rarely, even more).
Focal length is not the same as the distance of the camera's focus. If a camera is properly adjusted, then an object in focus at maximum zoom should be in focus at all zoom levels.
In fact, standard proceedure in studio settings is to focus a camera in an extremely tight zoom of its subject's eyes, then zoom back out to the desired shot. Zoom lenses on which this technique works are called parfocal. Some still camera lenses are not parfocal, but zooms used in TV virtually always are.
Extremes of focal length can produce optical distortion.
- Wide angles often produce:
- Barrel distortion, which causes straight lines to appear curved. If you are taking a picture of, say, a building, with a very wide lens, the center of the building is closer to you than the edges, which causes the center to seem to bulge out in the resulting picture. Lenses can be designed to minimize barrel distortion; such lenses are referred to as rectilinear. However, this causes objects at the edges of the image to appear stretched out. (Such lenses are often used in architectural photography but rarely in TV or film because it makes people look weird.) Barrel distortion is taken to extremes in the Fish Eye Lens shot.
- Z Axis expansion. Movement and distances into and out of the screen appear exaggerated. In wide angle, a man running towards the screen apears to move faster.
- A deep depth of field.
- Long lenses produce:
- Z-axis compression. Objects in different planes can appear to be next to each other. Movement toward the camera seems slowed. In a tight zoom, a distant man running towards the camera seems to be standing still. (See Monty Python and the Holy Grail for an example)
- A shallow depth of field. (However, since the subject in a long shot is often focused at infinity, it usually doesn't matter. That's why the entire football team can be in focus at once.)