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be necessary to provide a measure of photographic system 
performance by resolving power or some equivalent which 
indicates what size of detail can be seen in the photo 
graph, since we have no intuitive means for translating 
the MTF into visual image quality. 
The present status of the application of transfer 
functions to aerial photography is briefly indicated in 
the following paragraphs. Detailed discussion of any of 
the topics is beyond the scope of this report. 
Lens Design 
It is now common practice for the lens designer to 
calculate MTF's and use them to choose an optimum design 
before any glass is worked. The calculations are based 
on ray traces derived from design data, and the effective 
implementation of the method depended on development of 
techniques for weighting the MTF’s at different wavelengths 
to match the effective spectral sensitivity of the emulsions 
used. Because lens performance is still commonly specified 
by resolving power, the MTF must be converted into resolv 
ing power for the final optimizations. A technique for 
this conversion via emulsion ’’thresholds” is described in 
Appendix 3* The designer does, of course, take note of 
the whole MTF in making his final choice, but the ability 
to predict resolving power with reasonable assurance saves 
a great deal of time. 
Emulsion MTF 
The emulsion MTF has to be determined indirectly by 
methods discussed in Appendix 2. It differs from the MTF 
of the lens in that there are more opportunities for the 
results obtained to depend on the method of determination. 
Also, the emulsion MTF can vary with exposure, whereas the 
lens MTF behaves as a fully linear operator. Nevertheless, 
representative emulsion MTF's can be obtained and provide 
useful information. In system design for original aerial 
photography it is now common to work with MTF's up to but 
not including the emulsion, converting the overall system 
MTF into resolving power via a ’’threshold”, (see Appendix 3) 
which includes the MTF along with other emulsion character 
istics. This is quite logical, because emulsions are avail 
able in a limited number of types having standardized char 
acteristics which the designer cannot vary with the same 
freedom as, say, the lens design can be varied. For the 
study and comparison of emulsions as such, and for studies 
of printing procedures, the MTF is of course a valuable 
tool (70). 
Other System MTF's 
All aerial photographic systems have some degree of