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Of photographic materials. The problems are of the same order of 
difficulty as in obtaining agreement in sensitometric tests. Care is 
necessary to standardise processing conditions, the same type of emul- 
sion must be used throughout and each batch given standardising tests, 
exposure must be exactly controlled, and the same illuminating conditions 
used for the test-object. When al| this has been done the standard of 
judgement of individual observers even under controlled viewing condi- 
tions, and the definition of the end-point to be adopted, especially in 
presence of spurious resclution, remain as obstacles to easy standardisa- 
tion. The manufacture of test-objects, with the problem of maintaining 
dimensional accuracy at all Sizes, hes also been a practical difficulty. 
With sufficiently precise specifications, however, these troubles can be 
overcome. 
In the writer's experience such problems have caused more trouble 
than any lack of correlation between resolution tests and photographic 
quality as judged by the information content of the pictures, which 
the aspect receiving most discussion in the literature. It can be 
stated categorically that no single case has yet been met in which the 
order of merit of two systems as decided by resolution tests was inverted 
by practical air photographic tests. The resolution tests were of course 
low contrast. 
is 
Good evidence has been produced by other workers that resolution 
tests, particularly high contrast tests, do not correlate with sub jective 
impressions of sharpness, but these investigations relate mainly to pic- 
torial photography, motion picture photography and television, the picture 
in all cases being viewed at a distance. For air photography, where the 
picture is scrutinised under magnification for detaii down to the limit of 
resolution, it is felt that the resolution test (which after all is merely 
a formalised expression of the same thing) can provide a useful guide, and 
practical experience confirms this view. 
On the other hand, it is possible to agree with the point of view 
which emphasises that the limiting resolution figure does not provide 
the complete picture of a system's performance. The approach initiated 
by electronic engineers of expressing the performance of a lens in terms 
of modulation percentage as a function of line frequency is particularly 
attractive, offering as it does the possibility of specifying performance 
at frequencies well removed from the resolution limit, Possibly two 
numbers, one expressing the frequency for say 6 db loss of modulation and 
the other the modulation at or near the limiting emulsion resolution, 
would adequately specify the performance of a production lens. Such a 
test method appears to be more amenable to standardisation than any test 
involving photographic operations. For complete specification of the lens 
performance the full modulation/frequency curve would obviously be re- 
Quired at every significant point in the field, and analogous curves for 
each emulsion of interest. The method would also have the advantage of a 
standardised way of specifying performance, with figures interchangeable 
between photographic and electronic recording systems. 
Work has been started on 
these lines but no results are yet avai|- 
able to report.