( 
3 
diaphragm is reached only by a cone of r a ys slightly diverg 
ing in the object space. Magnification thus increases as 
object points recede from the optic axis. 
\Vh a t has been said se far about magnification applies to 
image formation in the optic axis. The distortion of a photo 
lens can be represented in a rectangular coordinate system, 
where distortion values (i.e. deviations of image point 
locations from those of mathematical perspective projection) 
are plotted vs. the associated angles of interference rela 
tive to the optic axis. The abscissa, representing angles, 
will necessarily be tangent to the curve in the coordinate 
origin, because by definition the distortion 
ds' = s' - c • tan , ( 3 ) 
s* denoting the distance measured between the principal 
point and the respective image point. 
Distortion in that sense may be called "absolute distortion", 
the imaging constant c corresponding to the Gaussian focal 
distance ( a long the optic axis). 
Photogrammetrie distortion 
Por the purpose of measurement, where geometrical dimensions 
are taken from the photograph, the concept of "absolute 
distortion" is impractical. Reference to a mean optical 
magnification, e.g. to the effect of demanding that distor 
tion maxima and minima are equal throughout the image field, 
is a more useful approach. This implies that the imaging 
constant c no longer applies to the Gaussian focal distance 
(in the optic axis), but to a suitably chosen angle of 
incidence ti of a principal imaging ray relative to the optic 
axis. Resulting from an imaging constant c thus defined, the 
statement ds' = 0 is true both for the optic axis and for the 
angle of incidence^. The distortion referred to such a