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2.2 Definitions. 
2.2.1 Fiducial Center: Point of intersection of fiducial axes. 
2.2.2 Principal Point of Autocollimation: The centre of the image formed in 
the emulsion plane by the camera lens from an incident beam of paral 
lel light which in the object space is perpendicular to the emulsion 
plane. 
note: The principal point of autocollimation is the type of 
“principal point” which can be determined most directly and 
accurately. Other definitions of principal point have been sug 
gested as partial remedies for inaccurate lens centering. All are 
somewhat artificial. If it is desired to use one of them for some 
particular application the test data are sufficient to permit its 
mathematical determination. 
2.2.3 Image Displacement: Image displacement is a vector quantity, being 
the displacement from the theoretical image point of an ideal lens of 
the same principal distance to the actual image point for the camera 
under test. It has as components radial image displacement and tan 
gential image displacement. Radial displacement is positive when it is 
outward from the principal point of autocollimation. Tangential dis 
placement is positive when it appears as counter clockwise in the 
image space to an observer looking toward the lens. 
2.2.4 Distortion: Distortion is an aberration affecting the position of images 
off-axis, caused by the fact that objects at different angular distances 
from the axis undergo different magnifications. Numerically, distor 
tion 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. Distortion is purely radial and is com 
pletely symmetrical. For a perfectly made and perfectly centred lens, 
image displacement is equal to distortion. In the presence of small 
manufacturing errors, average radial image displacement will be approx 
imately equal to distortion. 
note: Distortion has long been the name for a theoretical lens 
aberration which can be only radial and must be symmetrical. 
There have been attempts recently to avoid adding other mean 
ings by finding another name for the asymmetrical displacements 
which occur in practice. 
2.2.5 Calibration 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.