principal distance[′prin·sə·pəl ′dis·təns]
The perpendicular distance from the internal perspective center to the plane of a particular finished negative or print, or of a drawing.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
A standard by which photographic print is measured in terms of the focal length of the camera used at the time of film exposure. It is said to be of the same length as the focal length. Such a standard applied to an enlarged, reduced, or shrunken photographic print is determined by the ratio of the enlargement or reduction.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved