(1) The perpendicular distance from the internal perspective centre of a projector to the plane, in the projector, of the photographic copy (finished negative or print). This distance is equal to the calibrated focal length of the camera that took the original photograph, corrected both for the enlargement or reduction ratio and for the shrinkage or expansion of the film or paper since the photo-graph was taken. It preserves the same perspective angles at the internal perspective centre to points on the photographic copy as existed in the camera taking the picture at the moment of exposure. This is a geometrical property of each particular, finished negative or print. It is also called the effective focal length. (2) The perpendicular distance from the internal perspective centre of the projector of a stereo-scopic plotter to the plane, in the projector, of that side of the diapositive on which the emulsion lies.