3 
The MTF contains no phase information and is applied to 
emulsions, or to lenses when phase effects are small, 
i.e. when the image is not strongly asymmetric* In 
many circumstances the phase effects are less important 
than the modulation transmission* For brevity in this 
report, MTF is used for all cases in which specific 
emphasis on phase is unnecessary* While this is not 
strictly correct for optical systems, OTF would not be 
correct for emulsions, and it was not desirable to intro 
duce new terms* 
4* Interpretation of Resolution 1.5 
It is desirable to clarify some points about the 
interpretation of Resolution 1.5 and its relation to the 
Working Group's views. 
The resolution mentions the MTF as "an objective 
method of measuring image quality". It should be apprec 
iated that the MTF does not measure "image quality", but 
measures physical characteristics of image-forming devices. 
MTF's are certainly derivable from images, but the quality 
of any image is by no means fully specified when we know 
the MTF of the imaging device, any more than it is specified 
when we know the resolving power of a system. Many other 
factors are concerned, such as the size of the detail in 
which we are interested, the task we have to perform with 
the image, the contrast of the image, the nature of the 
sensitive surface and so on* Following a suggestion made 
at the London Symposium, the term "Imaging Quality" is more 
appropriate in the photogrammetric context* It should never 
be overlooked that the only information directly provided 
by the MTF is the system's response at the imaging or expos 
ure stage to different spatial frequencies in its object 
plane* 
The second paragraph of the resolution envisages "an 
agreed method to be incorporated in the ISP recommendations 
on Camera Calibration". The Working Group's interpretation 
is that this refers primarily to a recommended method for 
measuring the OTF of camera lenses. However, the Working 
Group considers that it will never be desirable to recommend 
one specific method for this purpose* Several different 
methods are currently available, which employ quite diff 
erent technical approaches, but by virtue of the fundamental 
nature of the OTF they are potentially capable of giving 
equivalent results. At this time it is not desirable to 
recommend methods, but only to indicate precautions which 
must be observed if any method is to be satisfactory. 
Section 6 and Appendix 4 embody such recommendations.