5* General Applications of Transfer Functions in 
Aerial Photography 
From the point of view of aerial photography, the MTF 
differs from the classical resolving power test in the 
following ways: 
(1) It allows measurements to be made of the 
imaging performance of individual compon 
ents, free of the influence of other com 
ponents. 
(2) The measurements are made in objective 
physical terms. 
(3) Expression of the results as modulation 
versus spatial frequency gives a useful 
general impression of the system perfor 
mance over a range of sizes, as distinct 
from the limiting size, which is the only 
information conveyed by resolving power. 
(However, this carries the danger of a 
too-literal interpretation for any target 
other than sinusoidal. The size/contrast 
relationship must be calculated by Fourier 
transform methods if correct values are 
required)• 
(4) MTF's for any number of system components 
can be combined by direct multiplication, 
thus facilitating the design of systems and 
the analysis and understanding of system 
performance. 
(5) MTF's can be calculated for various compon 
ents of the photographic system, e.g. from 
lens design data or for known amounts of 
image movement. 
(6) The MTF relates only to the optical image 
that is effective during exposure; it takes 
no account of the factors such as gamma, 
granularity and human vision that enter into 
resolving power, and conveys no direct infor 
mation about the developed photographic image. 
Nevertheless, the MTF provides valuable links 
between physical and visually observed perfor 
mance, thus a MTF can be translated into resol 
ving power and a MTF can be derived from a 
photograph, e.g. via microdensitometry of edge- 
images. 
Thus the MTF is not to be regarded as a mere substitute 
for resolving power, but as a powerful tool for the analysis 
of imaging performance in fundamental terms. It will always