y, Which must give 
gins to droop with 
l' curves have been 
sensitometric con- 
a function of fre- 
e will serve as an 
  
solving power and 
alain. 
type, representing 
would have unity 
ough 0 on the “log 
use. Had the test- 
we been displaced 
ast level of 0.048 
eye) would inter- 
ring in mind that 
the region 50-100 
uch higher visual 
age is not main- 
smaller aperture, 
mit of its field, is 
inferior to (1) both 
visually and photo- 
graphically, in spite 
of its larger aperture. 
In general we can ex- 
plain all facts and 
apparent anomalies 
about lens perform- 
ance, given the C.T.- 
curves and necessary 
emulsion data. 
If resolving-power 
is the point of interest 
in studying the joint 
behaviour of lens and 
emulsion, then lens 
C.T. curves can use- 
fully be combined with 
emulsion threshold 
curves. Some exam- 
ples of the latter are 
shown in Fig.7. These 
were obtained by im- 
aging long-line sinus- 
oidal test objects on 
the emulsions at va- 
rious contrasts and 
determining the con- 
trast needed at dif- 
ferent frequencies for 
the developed image 
to be just visible. The 
lens used had practi- 
cally unity C.T. over 
the frequency range 
concerned, but small 
corrections were ap- 
plied. 
The emulsions span 
the range of sensi- 
tivities which have 
been used for air pho- 
tography, and all were 
given full development. 
(10 mins. D19 at 68° 
F). Intersections of 
the curves with the 
line “log C.T.= 0” 
gives the  high-con- 
trast resolution, inter- 
section with the log 
C.T axis gives the 
THE PHOTOGRAPHIC IMAGE, BROCK 
Fig.7. Threshold contrast curves for six emulsions. 
[9] 
~~ 
+ -1.0 
[^] 
[e] 
f 
- 
c 
o 
[9] 
Nr 
© 
oO 
- 
-20 
o 
HH 
U -10 
© 
O 
- 
- 20 
Fig.8. CT curves for 6" f/5.6 lens at high and low contrast, 
with threshold curves for standard high speed and slow high- 
13 
  
  
7 
/ 
  
  
  
  
N 
  
  
  
  
  
  
1,0 
LOG (LINES/mm.) 
2.0 
  
— 
| 
4 
  
| 
| 
  
M. 
  
EN 
7 
  
\ 
  
  
  
— 
  
  
  
  
10 
LOG (LINES/mm.) 
resolution emulsions. 
20