Examples of aerial mapping lenses 
  
I will give now some examples of how OTF-based imaging quality can 
be deduced and used in practice as a quality standard for aerial 
mapping lenses. 
Imaging quality of optical systems is limited by diffraction which 
is caused by the finite aperture, the f-number. However, lenses for 
larger image fields are mostly influenced in their imaging quality 
by the size and shape of aberrations in the image field. 
Modern aerial mapping lenses require, due to their extreme image 
field of 2 x #5 > a highly corrected lens as far as distortion, 
field curvature and astigmatism are concerned. To fulfill this de- 
mand, a relatively large number of elements is necessary, among 
these some meniscus-shaped lenses. Therefore, the system becomes 
sensitive to centering errors, and apart from a good design, care- 
ful fabrication is the prerequisite for a lens to show satisfactory 
imaging quality. The given modulation transfer functions of the 
used film as well as structure and low contrast of the most inter- 
esting object details restrict the limit of spatial frequencies to 
50 cycles per mm. Mostly, even 30 cycles/mm are a sufficient spatial 
bandwidth in practical use of today's aerial mapping lenses. Because 
of the low contrast of usual objects and further reduction of con- 
trast by the film, image motion and other factors, a bandwidth 
of up to 30 cycles/mm means that in this range the modulation trans- 
fer factor of the lens should be at least 0.40 or better 0.60. But 
we will see that this cannot be achieved over the whole image field 
even in case of modern lenses. Modulation is defined as 
qd -— J . 
max min 
m = 
+ . 
max min 
where J and d . 
max min 
test object. Formerly, this was also called ontrast. It is known 
from the Seidel theory that distortion increases with the third 
is the maximum or minimum intensity in the 
power of tangent w(w = image angle),field curvature and astigmatism 
with the second power. Thus, a large, well-corrected field requires 
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