special attention in order to reduce these aberrations. The reductior
and the knowledge of the distortion is of such an importance for
mapping lenses that for a long time excellent testing methods for
this kind of aberration have been developed. Field curvature and
astigmatism together with extra-axial spherical aberration (coma)
limit the imaging quality in the image plane which is given by the
position of film. Deviations of the tangential and radial field
curvature from a plane yield a decrease in imaging quality as de-
focusing does on the optical axis. Thus measuring the image cur-
vature of the lens under test is a useful means to evaluate OTF's
for those image angles where the smallest and largest deviation
from the given image plane is found. Deviations of the symmetry
of field curvature around the optical axis are due to centering
errors. Thus variations of these curves while turning the lens
around its optical axis help to find the extreme value of the OTF-
change over the field, that means, the variations of image quality.
Figures 1 to 5 give typical field curvatures for radial and tangent i
azimuths.
To show the close correlation of OTF and field curvature, Figures
1 to 5 give at the top the tangential (Jf) and the radial (=) field
curvature and in the lower part MTF-values (modulation transfer
function = amplitude of OTF) of a spatial frequency of 20 cycles/mm
for tangential and radial azimuths as a function of the field
angle. MTF is measured in the image plane of best modulation on
axis for full aperture. Figures 1 to 5 show this representation
for 5 different aerial mapping camera lenses of old and new design.
Besides the correlation of MTF and field curvature, one can see an
additional degradation of the MTF with growing field angle which
is caused by increasing amounts of coma.
Asymmetrical field curvature and MTF values for * and - field
angles due to centering errors may be detected and should be further
analysed by rotating the lens about its axis. The spatial frequency
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