9 
While these considerations apply to all lens MTF's 
used in photogrammetry, it is the opinion of the Working 
Group that attention should first be concentrated on the 
camera lens, which is the primary component in the photo- 
gramme trie system. 
7. Measurement of Transfer Functions of Photogrammetrie 
Lenses 
Numerous methods have been proposed and are used for 
measuring the OTF and MTF of photographic lenses. It is 
not the purpose of this report to analyze specific tech 
niques in detail, since they are described in the volum 
inous literature. (See references) 
In principle, most of the methods are based on (a) 
measurement of the intensity distribution in the spread 
function and mathematical transformation, or (b) direct 
measurement of modulation and phase in images of sinus 
oidal targets, or (c) convolution of sinusoidal masks with 
the enlarged image of the spread function. In all cases 
accurate microphotometry at low intensities is involved. 
This section discusses basic matters and problems which 
experience has shown to be important. 
It will be appreciated that since the MTF is expected 
to convey more, and more fundamental, information than a 
resolving power figure, its measurement must be performed 
with correspondingly greater sophistication if the results 
are to be relied upon. Though the data is expressed in 
frequency terms, the operations are essentially equivalent 
to an analysis of the intensity distribution in the spread 
function *, long recognized to be a difficult task. 
Measurements showing modulation as a function of spatial 
frequency can be obtained without great difficulty, but it 
is essential to realize that the results are not necessarily 
superior to resolving power merely because they have been 
obtained from an aerial image and are expressed in spatial 
frequency terras. Unless the apparatus used has been thor 
oughly qualified, and its precision and accuracy are known, 
(and the interpretation understood) the general introduct 
ion of MTF measurements might well cause greater confusion 
than exists through the use of resolving power. 
Since the spread function of a lens differs for every 
focal plane, field position, aperture, wavelength of light, 
and orientation of target, all of these must be identified 
for each quoted result. These and other requirements are 
discussed in succeeding paragraphs. 
* As just mentioned some techniques do measure the spread 
function directly and derive the MTF by mathematical trans 
formation.