(175; 
plotted as a function of the focal setting which provided the minimum con 
trast threshold value for that particular size of symbol. 
One of the major projects over the past years at our laboratory has been 
assessing the criteria for recognition and detection of photographic detail. The 
techniques of this type of experimentation have been previously discussed 7 ’ 8 . 
If the significance of the experiment may be compressed in one sentence, it 
might be stated that we hold that photographic quality in fields of technical 
photography is determined by the amount of information which the photo 
graphic interpreter can extract from the picture—that, given two photographs 
of the same object, the one which enables the photographic interpreter to 
detect more symbols is the better quality photograph. The second experiment 
of interest in today’s discussion was conducted with this same lens to assess the 
probability of detecting symbols on the photographic material. For this ex 
periment, simulated one-foot objects were photographed at various scales. 
Scale was then plotted as a function of the focal setting which provided the 
maximum probability for detection of the symbols. The results of the two 
experiments are compared in Figure 6 
wherein good agreement is evident 
between the focal setting which pro 
vided minimum contrast threshold for 
a given image size, and that which 
provided peak probability of detection 
of the corresponding size of symbol. 
We therefore conclude that, be 
cause the threshold characteristics in 
tersect as focal setting is changed, as 
shown in Figure 4, and because of the 
establishment of the fact that the focal 
settings for minimum contrast thres 
holds and peak detectability are in 
agreement (Figure 5), (a) any judg 
ment of relative performance of pho 
tographic systems based purely on a 
resolution number derived from a 
high-contrast test object may well be 
misleading, (b) any judgment of oper 
ational focal setting of the photo 
graphic system based purely on a reso 
lution number derived from a high- 
contrast test object may well be incor 
rect. More meaningful approximations 
have been suggested with an eye towards simple laboratoiy pioceduies, howe 
ver, judgments based on any resolution or sharpness considei ation deal only 
with a single size of symbol. 
Fig. 6. Focal Settings as a Function 
of Scale. 
Our interests in airborne operation deal with a range covering man) siz^s 
of symbol. For this reason, an absolute rating which must be established should 
be based on a function which weighs the quality over the range of symbol 
contrasts and sizes subject to consideration. The area over the contrast thresh 
old characteristic of the system is suggested as being this type of function.