4 
mean optical magnification can be obtained with sufficient 
accuracy from the distortion diagram described by a corres 
ponding rotation of the coordinate system. 
Demands placed on a photogrammetrie camera lens system for 
the least possible distortion mostly imply the conception that 
in this c^se the object-side and image-side cones of rays should 
be congruent within the accepted tolerance. This constraint, 
however, is unnecessary. Let us, generally, denote the angles 
of incidence of a principal ray relative to the optic axis by 
^and their corresponding angles in the image space by ^. If 
we put the condition 
tan 
tan IT 
k - constant, 
(4) 
the image distance from the principal point H* in the image 
plane to a particular image point will become 
s* •» c • k • tan T *. ( 5 ) 
It can be seen that congruent cones of rays in the object 
and image spaces constitute a special case only of this gene 
ral relationship, with k = 1. The absence of the necessity 
to have both cones of rays congruent is very significant 
for extremely wide-angle photogrammetric photolenses. In 
this case, too, the optical designer is enabled to provide 
for the imaging rays to strike the photographic emulsion at 
the smallest possible angle to the perpendicular of incidence. 
Distortions as defined in the preceding two chapters can be 
determined only for an agreed wavelength of the light used 
for imaging. It may differ appreciably in other wavelength 
ranges (e.g. in the frequently used near infrared), depending 
on the 3sns design.