in which 
C diameter of admissible disc of confusion (same units as f); 
N relative aperture. 
We have found a value of C as large as 0.1 mm to yield acceptable results for 
precise mensuration. Figure 2 shows the values of ju to be expected for various 
focal lengths and relative apertures when this value is adopted for C. The figure 
shows that for most situations of practical interest Jj can be expected to range 
between .02 and .05. 
By substituting the above expressions for s, and s, into (9), we obtain upon 
algebraic simplification 
a result, interestingly enough, that is independent of s. 
Ww 
.20 
  
7 
or 
pe m 
> 
  
  
  
ee" 
Len 
| eere 
  
  
  
  
  
  
  
60 80 
RELATIVE APERTURE 
Figure 2. Values of the parameter u as a function of focal length f and relative 
aperture N for an adopted disc of confusion of diameter C = 0.10 mm.