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n error for co-ordinate measurements horizontally and vertically can be con- 
m d to be a measure of the quality of the photograph (12). In the latter case 
M determination must be made of the possibilities of identifying the 
m details or of analyzing the contents of the photograph (13). It is not cer- 
em that the quality assessment gives the same result in both these cases or that 
agreement is obtained with the line frequency measured in one or the other way 
in a test photograph. 
It would naturally be simplest if the quality of the photograph could be de- 
termined for any photograph with the aid of, for instance, micro-densitometer 
measurements according to Mac Donald (14). If this is not possible, some type 
of test target and a certain definite method for the evaluation must be used. 
The concept of “acutance” is naturally of interest in this connection, since 
in interpretation various contours on the aerial photograph are frequently followed 
If the acutance is multiplied by an exponential function of the resolving limit 
for high contrast, Higgins and Wolfe (15) have obtained a value which in model 
tests for aerial photographs has given good agreement with ocular assessments 
of the quality of the photograph. 
In aerial photography, the small contrasts are generally of the greatest interest, 
which is shown i.a. by Carman and Carruthers (16). In terrain where woodland 
predominates as in Sweden the object has in addition intrinsically low luminance 
values, furthermore, its size is small and the contrasts that are of greatest interest 
are very small. Luminance differences with a logarithmic value of 0.05 occur 
here according to examinations made by Backstrom and Welander (17). 
Since haze reduces the small contrasts mostly in the shadows it is probably 
that relatively dark low-contrast targets are most suitable, at least in our country. 
A test target for variable contrast has been designed by the National Bureau of 
Standards (7). It is modelled on the well-known Sayce-target, in which the 
density of the lines in the upper part of the target is 1.5 and then drops to zero 
in the lower part. It appears that the target has been used only for laboratory 
tests. It should be possible to use this type or some similar type of test target 
also for aerial photography, provided that approximate line frequency for some 
different contrasts is known, as in that case it is possible to keep the size of the 
target within reasonable limits. Photographing must then be done from several 
different altitudes, for instance with intervals of 100 m, some altitude always 
suiting the frequencies of the test target. 
That both the line frequency and the contrast transmission must be used for 
testing lenses has been shown i.a. by Ingelstam and Lindberg (18, 19). Attempts 
to find the relationship between the contrast-transmission function and the in- 
formation of the photograph have later been made by Ingelstam and Hjelm- 
strom (20). 
The great problem nowadays is to determine for actual aerial photography the 
quality of the photographs obtained from various altitudes, weather conditions, 
Cameras and aircraft. This refers primarily to the negative of the aerial photo- 
graph. On the other hand, it is difficult in serie production to keep an 
absolutely even quality of the positive prints. For this reason also an objective 
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