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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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