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principal point[′prin·sə·pəl ′pȯint]
The intersection of a principal plane with the optical axis.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
As it relates to photography, it is the foot of the perpendicular to the photo plane through the perspective center. Generally, it is determined by the intersection of the lines joining opposite or diagonal collimating or fiducial marks. See also collimating marks.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved