Thus the cost must be held within a value approachable by the users. The way leading 
to this result passes through the removal of all devices not necessary to the good working of the 
camera, the selection of the most simple design, the employment of sensitized material not to 
be flattened inside the camera, 
The plate cameras have all these features : the film running is eliminated, the plates 
are flat, cameras are small and light having the same image size as the film cameras and the 
final accuracy of the survey is not affected by the image distortion during the photographic 
developping process. 
4, Solution of some problems for 13 x 18 cm plate cameras 
  
According to the subject of the above chapter, we have established what is the camera 
type better answering to the requirements of the photogrammetry for not topographic purposes. 
The camera having 15 cm focal length, 13 x 18 cm plate not yet satisfying all needs, but 
it has quite all the features for an universal use, plotting facilities and measurement accuracy. 
While designing the camera several and complex problems occurred. A part of them 
present some interesting points which we thought worth of mention. 
4.1. Principal rays distortion at infinity and close range 
  
This idea is now explained through a more simple way, considering the reader informed 
about the laws of Gauss optics. 
Let us consider a thin lens L with focal f, having entrance pupil Z obtained for instance 
through a mechanical diaphragm (see Fig. 15). 
  
  
  
  
  
  
Fig. 15 - Infinity and close range photography 
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