EN: Principal ray
A light ray which enters the lens at an angle at a point other than the optical axis point and passes through the center of the diaphragm opening. Principal light rays are the fundamental light rays used for image exposure at all diaphragm openings from maximum aperture to minimum aperture.
Canon glossary EU
AKA: Principal Filming, Principal, ShootThe filming of major or significant components of a movie which involve lead actors.
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The point from which the focal length of a lens is measured. The principal point of a simple lens is located at the center of the lens. Compound lenses have two principal points, the location of which cannot be determined by appearance.
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Principal point (nodal point)
The focal length of a thin, double-convex, single-element lens is the distance along the optical axis from the center of the lens to its focal point. This center point of the lens is called the principal point. However, since actual photographic lenses consist of combinations of several convex and concave lens elements, it is not visually apparent where the center of the lens might be. The principal point of a multi-element lens is therefore defined as the point on the optical axis at a distance equal to the focal length measured back toward the lens from the focal point. The principal point measured from the front focal point is called the front principal point, and the principal point measured from the rear focal point is called the rear principal point. The distance between these two principal points is called the principal point interval.
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