the definition of image, whereas the constant c, refers to the size of image, ie. to the 
radial distance corresponding to a certain object angle. A clear distinction of these two 
different concepts is the more necessary that also in photogrammetry, Bildweite in the 
optical sense occurs at times (as in the theory of rectifiers). 
The qualification “calibrated strikingly expresses the freedom in choosing the 
constant ¢;. This is an advantage but no absolute necessity. For the rest, however, ali 
designations formed with “length”, “distance”, and “-weite” for expressing the factor 
€, suffer from a basic drawback. This is that they interpret concept geometrically and 
therefore evoke the question: From which point of the lens must this distance be 
reckoned? Of course, such a point may be stated. To do so, however, neither leads (at 
any rate in the event of distortion) to the back nodal point nor to the exit pupil, nor to 
any other known point, and therefore means nothing but a geometrical representation 
of the factor €;, Which does not answer the question just put but merely expresses it in 
another way. This grows particularly clear when it is remembered that the factor Cr 
may be fixed as one likes without attendant change in the camera. 
Because of these reasons, the expressions “Kammerkonstante” and “Bildkonstante” 
(camera constant and image constant) were proposed for this factor in Germany and 
have been entered into the draft of the new standards sheet [4, 9]. These terms clearly 
express the nature of this concept. The factor c; has no geometrical significance but is 
an analytically defined calibration or scale ratio factor. The word “Konstante” means 
that in the equation: 
v=¢c,.tanz + dr 
the factor c, remains constant irrespective of changes of the variable quantities, and 
especially of object angle 7. The term “Kammer” indicates that this constancy refers to 
à certain camera, i.e. to a certain spatial arrangement of the lens and the image plane. 
As soon as this arrangement is altered, as happens in focusable photogrammetric came- 
ras, the constant must, of course, also change. The fitness of the term “camera constant" 
is not denied by the fact that its numerical value is arrived at by a more or less arbi- 
trary calibrating process. That, indeed, is true of any instrument constant. In spite of 
its arbitrariness, this numerical value, once it has been determined in any way, is 
regarded as a fixed and permanent characteristic of the camera, and is usually stated 
on a tablet on or in the camera. These are the reasons which have led to the designation 
"Kammerkonstante" in Germany. 
The definition of the camera constant purposely limits itself to the basic and leaves 
open the individual details of numerical fixation. In partieular, the process recommend- 
ed, of assuming zero distortion for the so-called zero distance, by no means implies 
that any particular kind of compensation of distortion is prescribed, for the magnitude 
of the zero distance is completely arbitrary. It is merely shown how the magnitude of 
the camera constant can be unequivocally fixed by a simple measuring directive. — 
Now and then it is recommended to make the maximal positive and negative values of 
distortion inversely equal by a suitable choice of the camera constant. Whether this 
method is to be recommended or not shall not here be discussed. It suffices to say that 
for defining the camera constant, this condition is too narrow and the measuring direct- 
ions are too complicated. Anyone who wishes to apply this method of compensation can 
easily name the zero distance at which the distortion curve passes through zero between 
its positive or negative maxima, and thereby enable anyone to check the camera con- 
stant in a simple manner. In a similar way, however, any other desired course of the 
distortion curve, at which the zero distortion value is again attained outside of the 
principal point can be fixed by the choice of the zero distance. 
SUMMARY. 
The definitions of photogrammetrical fundamental concepts rest on the relation 
    
  
  
    
  
  
  
  
   
    
   
    
   
     
   
    
   
   
   
  
  
  
  
   
    
     
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