SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
of the methods described here has, however, given results which are reported 
as reliable in aerial photography. 
The transfer functions 
The photo interpreter is obviously interested in small details as well as in 
larger objects. The contrasts between object and background also vary from 
high to low, although small contrasts are the most common. Concerning the 
contrasts reproduced, the camera system acts so that large objects in the scene 
are transferred to the photograph at the same contrast as in the original. The 
transfer is here said to be 100 percent. However, the smaller the detail, the more 
contrast will be lost in the transfer from object to photograph. For the smallest 
of all details in the scene, any contrast between detail and background in the 
photograph cannot be obtained. The transfer is said to be zero percent. For an 
aerial photograph the contrast reduction may begin even with details of the 
size of about 0.5-0.2 mm. 
Density 
u b 
2.0 
sample 
0 10 20 30 40 *B 50/* 
g* - 
* N 
_2 
Distance 
Acutance = 
1952 
Acutance = 
(mod if ied) 
2 
D b D a 
1956 
Transfer 
50 
f/5,6 
f/16 
Frequency 
0 10 20 30 40 50 l/mm 
Focus for max. transfer at 40 l/mm 
Image centre, filter 5300 A 
Fig. 2. 
Transfer function for the 6" lens Ag 38 
Fig. 1. The acutance according to Jones and Higgins. The diagram represents a knife-edge 
exposure and the microdensitometer trace D, across the developed image. The straight line E, 
between points A and B and the hypothetical dotted curve F, represent traces having the 
same average gradient as curve D. However, curve D provides an image which appears 
sharper to the eye. 
The ratio of the contrast in the image and in the original for details of 
different sizes is commonly represented as a diagram, the transfer function. 
The principles of the T-functions are applied in optics by, among others, Ingel-