APPENDIX | 
DEFINITIONS 
Focal Plane 
The plane in which the emulsion is held in the camera. |n a camera 
having a register glass, it will be the outer surface of this register 
glass. 
2. Fiducial Centre 
The point of intersection of the fiducial axes, or the centre cross 
in a camera having a register glass. The fiducial centre is adjusted 
during manufacture or calibration to coincidence with the centre of 
symmetry (see Clause 10). 
=. Origin of Measurements 
  
Measurements in the focal plane are taken with the fiducial centre 
as origin. Angular measurements in the object space are taken from the 
parallel beam of rays which is formed into an image on the fiducial centre. 
4, Principal Distance 
A camera constant, having the dimension of length, which determines 
the scale of the photography. It is approximately equal to the equivalent 
focal length of the lens. A precise value cannot be assigned to the 
principal distance without making some assumptions regarding the distor- 
tion (see Clause 9). 
5. Distortion 
Distortion is a vector quantity, being the displacement (from the 
required position) of the actual image point. |t has as components, 
radial and tangential distortion. 
6. Radial Distortion 
In a lens having radial distortion, the distance r of an image 
from the fiducial centre will be related' to the angle 6 subtended by 
the incident beam in the object space by the equation 
r = f tan 6 + D (1) 
where f is the principal distance 
D is the radial distortion. 
The distortion figures obtained from this eouation depend upon the 
principal distance chosen. 
    
    
    
     
    
     
   
    
   
    
     
      
    
    
     
   
     
    
    
    
   
      
    
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