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.
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.