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4.1 Averaging with respect to spatial frequency 
The spatial frequency at which the product of the lens and 
film MTF has fallen to a predetermined value has been used 
as a criterion for analysing subjective judgments of picture 
quality in aerial photography. More recently several authors 
(5-16) have favoured MTF curves. In fig.3 the area between 
the lens MTF and the threshold contrast curve is indicated. 
It can, however, be shown that for practical purposes the 
area between the MTF and the abscissa up:to the limiting 
frequency is adequate for comparison. The threshold curve 
in fig.l assumes a 0.03 threshold contrast of the eye and 
the use of Agfapan Emulsion (11). 
To allow for low-contrast situations in aerial photography 
the MTF can be scaled to any object contrast - a frequently- 
used low contrast is 0.23 at O Lp/mm. For comparison purposes, 
the intersection of the threshold with the MTF curve leads to 
the resolution as indicated earlier. 
The area below the MTF curve requires weighting with the areal 
coefficient to which the chosen image point belongs. For direct 
comparison of different Systems, the area can be normalized 
with respect to the diffraction-limited MTF for instance. For 
simpler test procedures the MTF for two or three selected 
spatial frequencies could be considered. 
Regularity with respect to tangential and sagittal MTF 
To take into account regularity with respect to tangential 
and sagittal MTF, the limiting frequency (Rg) for integration 
can be defined by the intersection of the threshold with the 
lower MTF curve. The same limiting spatial frequency Rg applies 
for the integration of the higher MTF curve. The geometrical 
mean of MTFsag» (Ts), and MTFran: (Te), is considered in further 
comparisons (dotted line in fis.3). 
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