NOTES ON CALIBRATION OF CAMERAS FOR AIR SURVEY 
by 
R. W. Fish, M.Sc., A.R.C.8.* 
  
SUMMARY 
The basic concepts of camera calibration are reviewed and the 
definitions of camera parameters in the proposed International Standard 
are discussed. A method of reduction of measured values obtained during 
calibration is described and the importance of setting the central fidu- 
cial mark of the camera, to the centre of symmetry of the lens, is 
emphasised. The distance between the centre of symmetry and the princi- 
pal point of auto-collimation is shown to be a measure of the tangential 
distortion. Proposals for modifications and additions to the draft 
International Standard are made. 
‘ INTRODUCT ION 
For some time now it has been apparent that there are considerable 
differences in terminology between various photogrammetrists, and between 
photogrammetric texts. Thus the standard definitions of focal length, 
principal distance, principal point etc. are far from being unanimously 
and uniquely defined. [It is believed that these differences arise largely 
from the use of simple optical concepts whose meanings have been extended 
far beyond their original scope. These concepts are of value to optical 
designers working on paper during the design stage, for their lenses are 
always "perfect" as regards construction, centering, etc. When we come 
to calibrate real cameras, however, the image does not obey these simple 
laws to the accuracy necessary to the surveyor, i.e. 5-10 microns. When 
we reach this order of measurement we find that many lenses are asymmetri- 
cal, that terms such as optical axis, nodal point, principal focus, etc. 
are difficult, if not impossible, to define, and that conventional optics 
are no longer sufficient. Calibration of a camera must then be considered 
from a photogrammetric viewpoint. 
In practice, a laboratory is presented with the camera to be cali- 
brated, and has to rely entirely upon its own measurements, no design 
data being of value since the lens and optical. system will depart signi- 
ficantly from the lens designer's prescription. Only the focal plane has 
been defined either by a register glass or a datum surface against which 
the photographic emulsion is held. How this focal plane has been 
* Royal Aircraft Establishment, Ministry of Supply. 
  
  
  
  
  
  
  
  
  
  
  
  
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