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7 
position of images off-axis, caused by the fact that objects at different angular 
distances from the axis undergo different magnifications. Numerically, theoretical 
distortion is the displacement from the theoretical image point of an ideal lens of 
the same principal distance to the theoretical image point for the actual lens 
design under study. It is positive when it is outward from the centre of the field. 
Theoretical distortion is purely radial and is completely symmetrical. For a 
perfectly made and perfectly centred lens, measured distortion is equal to theore- 
tical distortion when the position of the image plane and the principal distance 
are the same in both cases. 
Calibrated Principal Distance or Calibrated Focal Length: An adjusted value 
of the principal distance, chosen so as to distribute the distortion in the manner 
best suited to the plotting conditions to be employed. The report shall state the 
way in which the calibrated principal distance has been chosen. 
Average Radial Measured Distortion: Average radial measured distortion at 
any field angle is the average of all radial measured distortions occurring at that 
field angle. In practice it will usually be adequately determined as the average 
of the radial measured distortions found for four or more points equally spaced 
along the circumference of a circle centred at the principal point of auto- 
collimation. 
Principal Point of Best Symmetry: 'The principal point of best symmetry is a 
point near the principal point of autocollimation chosen so that when it is used 
instead of the principal point of autocollimation as a new origin for distortion 
measurement it makes the largest absolute difference between new radial 
measured distortion and new average radial measured distortion as small as pos- 
sible along each diagonal of the image format. 
Focus Setting 
Cameras shall normally be supplied for calibration and for use focussed to 
give best average photographic resolving power. A different focussing criterion 
may be used only if it is clearly justified by a user requirement and is stated in 
the report. Calibration shall be carried out in the actual focal plane of the camera, 
that is in the position the emulsion surface, if it were a true plane, would assume 
in service. If in any type of camera the intended shape of the emulsion surface is 
not a plane, the calibration shall be carried out in the intended surface, or by a 
method which produces equivalent results. 
Target 
A high contrast target shall be used. 
Collimators 
Each target shall be placed at the focus of a collimator. A single collimator 
which is set at various angles relative to the cameras may be used, or an array of 
collimators providing all the necessary field angles. Collimator lenses shall be of 
sufficient quality and focal length shall be focussed accurately enough that dis- 
placements of the camera lens which permit of it being entirely filled do not in- 
troduce significant errors in measurement. 
  
NOTE: Spherical aberration of the collimator objectives or small focussing 
errors could introduce significant inaccuracies. Such effects should be 
  
checked carefully.