Carman-Carruthers data, Figure 4, with the relative importance of each con 
trast level being taken as the relative frequency of occurrence of symbols at 
that contrast. 
If we assume that the minimum flare condition corresponds to the mini 
mum contrast threshold condition for a particular image size, then there are 
two convenient laboratory means of approximating this best focal setting. 
One is to determine the maximum resolution setting for a low-contrast test 
object. This means we deal with the approximate center of the range of 
symbol sizes which we expect to encounter in aerial photography, rather than 
the fine detail limit. 
The other is to determine the minimum flare setting for a pinhole image, 
which in the case of photogrammetry should be about 1/30 mm in diameter. 
6' INCH METROGON 
© FOCAL SETTING FOR MAXIMUM RESCLUT ION 
© O.I7mm INSI0E CURVE I 
(3) 0.40mm INSI0E CURVE I 
A B 
Fig. 5. Contrast Reduction (A) and Object Contrast Thresholds (B) for Periodic Detail. 
Flansen 4 has shown that observers, in rating photographs in terms of 
sharpness, arranged them in a manner that correlated with minimum flare 
settings rather than resolution settings. The recent work of Higgins and Jones b 
has shown that the focal settings which, by subjective judgments, provide 
maximum sharpness do not necessarily coincide with the maximum resolution 
settings. In this work they have gone on to find the nature of the physical 
stimuli which provide the sensation of sharpness. In all this work, however, 
one discovers that the maximum sharpness judgments depend upon the con 
dition of observation, that the focal setting which provides the visual im 
pression of maximum sharpness for unaided vision is not necessarily the focal 
setting which provides maximum sharpness when the observation is made 
under two- or three-power magnification. Thus we extend our previous 
assumption that minimum flare and minimum threshold conditions are coin 
cident. We now imply that, for a well corrected imaging system, the focal 
setting which provides a minimum contrast threshold for the particular size 
of symbol under observation is the setting which records that same size symbol 
at maximum sharpness. 
Let us now examine our position. An experiment was conducted which 
has now been reported on 6 . In this experiment, contrast reduction charac 
teristics of a high-quality 8-cm Zeiss Tessar were examined. Symbol size was