5
CURVATURE CENTRE
Fig. 1.2:1. The principles of imaging the horizon from an airplane. The effects of the atmospheric refraction and of the
contour elevation are indicated. See table 1.2:1 (the Appendix).
of exposure. The discrepancies between the central and the orthogonal projection will be functions of
the flatness deviations and the distance from the principal point of the photograph. In fig. 1.3:1 some
causes of the lack of flatness of the recorded image in the camera are schematically shown. The various
factors have been tested in practice and the results of the test samples will be shown below.
1.31 The Supporting Plate in the Film Magazine
This plate is of basic importance for the flatness of the film in the moment of exposure. Special atten
tion is paid to the quality of the supporting plate in the camera factory. Leading camera manufacturers
have stated that the tolerances of flatness deviations in the factory are di 7 ¡tim or less.
Investigations of supporting plates in practical use have been made regurlarly at the Geographical
Survey Office of Sweden. The plates were measured with measuring gauges supported on surface plates
of high quality. Adjustments of the discrepancies according to the method of least squares indicated
root mean square values of remaining discrepancies of about 4 ¿¿m. The radial distortion effect of the
deformations were computed for the principal distance 153 mm from individual residuals combined into
circles around the central point of the plates. The results from three supporting plates are shown in
fig. 1.31:1. A significant radial distortion effect is found which may be reduced to some extent, however,
by possible pressure effects in the moment of exposure. It would be desirable to measure the flatness of
the plates under the actual pressure.
From the multicollimator tests discussed and shown in section 1.61, different radial distortion effects
were found in glass plate and film negatives, probably caused by flatness variations. See fig. 1.61:5
(the Appendix).
