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principal plane[′prin·sə·pəl ′plān]
For a quadric surface, a plane that passes through the midpoints of all the chords perpendicular to it.
Two planes perpendicular to the optical axis such that objects in one plane form images in the other with a lateral magnification of unity.
The vertical plane passing through the internal perspective center and containing the perpendicular from that center to the plane of a tilted photograph.
A vertical plane that contains the principal point of an oblique photograph, the perspective center of the lens, and the ground nadir. It is the vertical plane containing the optical axis.