6 
radial dislocation by intentionally tilting the taking axis 
toward the object (or the projection camera of a plotting 
machine in the course of relative orientation). Asymmetric 
deformations of the distortion model in the photograph are 
caused by decentrations in the optical system and tilting 
of its elemeAts. Geometric-optical computation based on 
the assumption of such asymmetric point dislocation discloses 
the existence of tangential components in addition to radial 
ones* As a consequence, straight object lines may appear as 
curves in the image plane even if they pass the image centre. 
Direct compensation of this tangential deformation effect 
in restitution is possible only if analytical techniques are 
employed. To account for both dislocation components, Ameri 
can publications have presented a simple model concept based 
on the fact that an ideal lens with a waek prism put in front 
of it can dislocate image points in a similar manner. This 
"thin prism theory*' permits to derive a mathematical rela 
tionship between the radial and tangential components, and 
in accordance with practical experiments on optical systems 
it was found that the tangential share should be smaller 
than the compensable radial distortion, investigations carri 
ed out by G. tfiirtz (G.D.K.) on modern, sophisticated high- 
-performance lenses, however, did not generally confirm the 
thin prism concept, which is supported mainly by P.K. 
v/asher (USA). 
Affine deformation 
Prom the effective image point dislocations it is possible 
to isolate affinity errors in addition to asymmetries. 
Affinity errors make -equal distances in the object appear 
contracted or expanded in the image depending on their azimuth 
position, so that a circle, e.g., would form an elliptic image. 
This error is possibly due to deviations of optical surfaces 
from the true spherical shape, caused by imprecise polishing 
or by warping under the influence of pressure exerted by