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The author precisely defines the term "distortion" as the 
rotation-symmetrical dislocation of image points between 
the mathematical model of perspective projection and the 
physical image formation model. On this physical distor 
tion, which is always rotation-symmetrical, is superposed 
a summation of unavoidable tolerances in the manufacture 
of optical systems, for which the term "deformation" is 
introduced. Further considerations deal with external in 
fluences on photogrammetric photography and lead to the con 
clusion that at the present state of knowledge no definite 
causes can be established for image point dislocations in , 
photographic image formation. Based on the assumption that 
model deformations are caused by imperfections of the 
photograph*s geometry, it is advantageous to compensate them 
by correction surfaces generated analytically or by analogue 
instrumental techniques from discrepancies at known control 
points. Some critical comment is given on the conventional 
solutions using optical compensation plates or reseau 
cameras. 
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