The principal point is the point on the image plane onto which the perspective center is projected. It is also the point from which the focal length of the lens is measured.
Near the principal point is the principal point of autocollimation (PPA). This is defined as the image position where the optical axis intersects the image plane.
The principal point of symmetry (POS), also known as the calibrated principal point, is the point on the image where a ray of light travelling perpendicular to the image plane passes through the focal point of the lens and intersects the film. In a perfectly assembled camera, the principal point of symmetry would be where the lines of opposing fiducial marks on an image intersect, also known as the indicated principal point (IPP). However, in most cameras a slight offset occurs. The perspective effects in the image are radial about this point.
The POS, PPA, and IPP values are usually specified in the camera calibration report.
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