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Definition of Focal Length
Answer provided by Alan Jones
Focal length is an optical term that we use all the time when talking about lenses. When we refer to a 50 mm lens or a 135 mm lens, we are referring to its focal length. Note that it is measured in millimeters.
By definition, focal length is the distance between the point in the lens where the light begins to diverge (the nodal point) and the focal plane where the film or CCD (Charge-Coupled Device) is positioned when the focus is set at infinity.
The focal length number gives you a lot of information about the appearance of images taken with that lens. It controls such characteristics as image size, angle or field of view, depth of field, and perspective. Understanding these relationships are important to choosing and using lenses.
Image size: The greater the focal length, the more magnified the image. When compared to a 50 mm lens, a 100 mm lens provides 2x magnification, a 200 mm lens provides 4x magnification, and so on.
Angle of view/field of view: The angle of view (or field of view) determines the amount of the scene which a lens takes in. A 50 mm lens is referred to as a normal lens because the angle of view is similar to that of the human eye. The greater the focal length, the narrower the angle of view.
Depth of field: Less depth of field is available as focal length increases for any given F-stop and camera-to-subject distance. For example, at F/11 the zone of sharpness (the hyperfocal distance) is about 8 feet to infinity for a 28 mm lens compared to about 150 feet to infinity for a 300 mm lens.
Perspective: The visual relationship of various subjects within the frame can be manipulated with focal length. A telephoto lens shortens the perceived distance between objects. With a wide-angle lens, however, objects seem farther away from one another than they really are.
References:
What focal length ranges are generally considered wide-angle, normal, and telephoto?
Camera Lenses
Depth of Field