CE PRE SEE EEE SES CPR A 
  
  
  
  
The MTF approach can lead directly to system analysis. The MTF 
of the total system is basically the product of the MTF of the 
lens, of the emulsion, and of the environmental disturbances. 
For user handling of the MTF curves derived from different 
image planes, apertures and field angles are not always con- 
venient. 
Different authors (2-8) suggested averaging the MTF with respect 
to spatial frequency. In an early proposal (3) three criteria, 
namely resolution, Sharpness and contrast, were considered. In 
later publications it was pointed out that only two of the 
previously-mentioned criteria are independent. In addition, r 
Some authors have taken the observer into account by evalu- 
ating the logarithm of the MTF (4,5,6 and 8), 
In aerial photography, faithful geometrical reproduction is 
more important than in classical photography. The interpre- 
tation of images is usually carried out in specifically-designed 
devices such as autographs, where image pairs recorded with 60 
to 80% overlap are analysed, and therefore high regularity of 
image quality over the whole field is required. 
Quality numbers based on MTF 
In most of the previous work on image quality, the MTF was (v 
averaged over the spatial frequency of the image. As indicated 
above, 
equal. On the basis of the above arguments it is proposed to 
incorporate the following criteria in an image-quality Speci- 
fication: 
the area below the MTF curve to a limiting Spatial frequency 
small variation with respect to the tangential and sagittal MTF 
- Small variation of image quality over the whole image field 
These three criteria will now be considered in more detail.