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BOUSKY 
The characteristic form of this ‘‘spreading out of light" is often 
called the spread function of the lens-film system. The form of this 
spreading may differ quite widely, but for a system of high resolution 
it should preferably have a sharp narrow tip. The wide ends or ‘‘skirts’’ 
of the spread function may interfere with the ability to record low 
contrast detail, while in most instances they may have almost a negli- 
gible effect on the high contrast resolution. 
Figure 9 shows approximate spread functions for an actual lens 
that were taken at two slightly different focal settings. Note that focal 
position B has a sharp tip with wide skirts, and that focal position A 
has a blunt tip with narrower skirts. 
This lens was tested for resolution at two values of target con- 
trast through a range of focal settings. The data are shown in Figure 
10. These data show that the sharp spread function position provides 
a relative resolution of 90 at high contrast, where it would be tested 
for acceptance; while its performance at a contrast of 0.2 (which is 
still reasonably high in actual use) has fallen to 38. On the other hand, 
the blunt spread function position, which would only test out at 72, 
provides a resolution of 50 at 0.2 contrast. This relative improvement 
could be even greater at lower contrast. 
This example indicates that the high contrast point at which a 
lens is presently tested for acceptance is not necessarily indicative of 
its relative performance under the conditions that obtain in actual aerial 
photographic usage. 
  
  
    
  
  
    
  
    
    
  
  
  
  
  
  
  
  
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20 05 10 15 20 25 
  
  
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-120 -80 -40 o 40 80 120 
FOCAL POSITION—MILLIMETERS 
FORMAT POSITION —- MICRONS 
Cone o MEASURED SPREAD FUNCTIONS MEASURED RESOLUTION AT TWO CONTRASTS 
AT TWO FOCAL POSITIONS _ _ FIGURE 10 
To explore the effect analytically, three idealized spread func- 
tions for which fairly simp.e mathematical expressions could be writ- 
ten were considered for lens systems. The forms of these spread func- 
Arc 
| Maruy 
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