One quality number for a lens or a camera can only be a summation 
of individual special qualities which have been set aside for the 
benefit of an integral consideration. In the literature, many pro- 
posals for quality numbers deduced from the OTF are discussed 
[6] = [10] In practice, these well elaborated quality numbers 
are not used, but for special lenses some producers indicate OTF- 
curves which characterize the image quality. 
Y 
In my opinion, this disinclination to integral quality numbers has 
two reasons: 
1. In order to deduce reliable quality numbers, the OTF curves 
must be known. In that case it is easier and more informative 
to publish these OTF-data directly. 
Mostly, a pretentious user of optical systems wants to know 
closer details than just a number in order to overcome the 
always imperfect optical system by skilful use and to reach a 
result which is optimal for his purposes. 
To describe these details, but at the same time keep their number 
small in the interests of clarity, is the most important task of 
testing optical systems. In the case of aerial mapping cameras 
this means besides format, field angle and aperture also evaluation 
of the distortion, the lateral colour error and imaging quality 
given by the OTF up to 40 cycles/mm for selected field angles which 
can be suitably selected when the field curvature and its fluctuatio 
by centering errors are known. Hence, only a modest number of 10 to 
15 OTF curves will provide all data of imaging quality. If desired, 
an internationally agreed upon procedure of summation to a quality 
number or figure of merit may then be applied for further reducing 
these data.