Focal Length

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Focal Length

For photographic purpose the focal length is a physical property of a lens (or mirror) giving the distance from the lens in which rays of light that fall parallel to the optical axis into the lens are focused to one point. The exact point from which the focal length is measured is the rear Nodal Point (the location of the 'exit pupil'). This point is located outside the physical lens for common wide angle lenses for single lens reflexe cameras.

For a better definition see Wikipedia: [1]

Together with the film or sensor format the focal length determines the image Field of View which is far more important for panoramic imaging.

Whereas for rectilinear (normal) lenses the focal length is the same throughout the image for fisheye lenses it is valid only for the image center.