determined is not a matter of calibration and should not form part of a 
standard on calibration. 
Considering now the measurements that need to be carried out, an 
immediate problem arises, for in general any measured values vill vary 
with the particular lens aperture chosen, the wavelength of light used, 
the type of emulsion and perhaps the properties of the film base. This 
type of detail can be standardised between laboratories and is a proper 
subject for an international standard. 
Having chosen the standard conditions, we need to think about the 
camera parameters to be measured. A little thought will show that none 
of the Gaussian planes or constants can be located, or measured independ- 
ently, to the accuracy that is required for modern cameras. Thus to take 
examples; a nodal point cannot be located with precision on a nodal slide 
bench because some image movement always takes place in the presence of 
distortion; a focal Length cannot be measured directly because one limi t- 
ing point, the nodal point, cannot be fixed and moreover one cannot 
directly determine the optical axis along which the measurement should 
take place. One point can always be determined with precision, the 
principal point of auto-collimation, which in a perfectly symmetrical 
lens and camera, defines the point at which the axis of symmetry meets 
the focal plane. Beyond this single point, we may only measure the 
position in the focal plane of images formed from parallel beams incident 
upon the lens at various angles. It must be emphasised that we cannot, 
or do not, measure the angles made by rays inside the camera - and such 
angles have little meaning anyway since the beams forming the images are 
converging. 
Calibration then can consist only of the determination of the re- 
lationship between the angles of parallel beams falling on the lens and 
the corresponding image position in the focal plane. The method by 
which the measurements are performed and the form of the relationship 
deduced between the measured quantities should be subject to an inter- 
national standard. : 
2, PHOTOGRAMMETRIC CONSIDERATIONS 
The photogrammetric process consists of two essential stages:- 
(a) In the aircraft, the camera is held over the target and 
records images in the focal plane at positions corresponding 
to the angular positions of ground targets relative to the 
camera. 
(b) The film is then used to derive the angular position of these 
objects, the angles allowing their ground position to be 
determined. 
It is possible to define the distortion of the whole system in such 
a way that concepts of focal length, etc. are not needed. 
Thus a standard set of angles and corresponding dimensions in the 
focal plane could be chosen, and the camera and plotting equipment both 
accepted or rejected according to their departure from these standard 
values. There are many practical difficulties in doing this; for example 
    
  
  
  
  
  
  
  
  
  
  
  
  
  
   
  
   
     
    
    
   
   
    
    
     
   
     
     
    
   
  
    
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