order to obtain a wide range of resolving powers, and also 
to reduce the amount of correction necessary to take account 
of the MTF of this lens. It is usual to employ microscope 
objectives for which the correction is negligible over most 
of the frequency range, thus the MTF operative in determining 
the threshold is effectively that of the emulsion alone. The 
MTF of the lens whose resolving power is desired, e.g. an 
aerial camera lens, will cut off at much lower frequencies than 
the microscope objective, and the effective MTF will be worse 
than that of the emulsion alone. Consequently, a sharp-edged 
target such as a three-bar* target will not have the same shape 
in the threshold image as in the image given by the camera lens, 
because the degradation of the harmonic structure of the target 
spectrum is less in the threshold image. The second error arises 
from the fact that the threshold is determined for three-bar 
images whereas the lens MTF is known for sinusoisal images. In 
general, the ”three-bar function” for a lens is substantially 
higher than its sinewave MTF, hence the experimental resolving 
power tends to be higher, perhaps 20% higher than the threshold 
crossing predicts. Nevertheless, within the accuracies of 
measuring resolving powers and MTF's the method has proved very 
useful for system design. 
Needless to say, the normal restrictions on resolving 
power accuracy due to exposure, development, etc., apply equally 
to thresholds. 
Figure 5 illustrates the application of the threshold 
principle and also the multiplication of MTF’s. Curve I is the 
MTF of a 6 inch aerial camera lens on axis. Curve 2 is the 
effective MTF of 25 microns linear image movement. Curve 3 is 
the product of curves 1 and 2. Since the plot is on logarithmic 
co-ordinates, the effect of using a low-contrast target, e.g. one 
of 0.2 modulation as in the Figure, is shown by moving Curve 3 
down so that it cuts zero frequency at 0.2 modulation. Similarly, 
the effect of using a low contrast target, e.g. one of 0.2 modul 
ation as in the Figure, is shown by moving Curve 3 down so that it 
cuts zero frequency at 0.2 modulation. The broken line is the 
threshold for Plus X aerographic emulsion. 
The ultimate resolving power for Plus X, which lies off 
the diagram, is about 120 cycles per mm. for a high contrast 
target. The high contrast resolving power for Plus X with the 
lens is seen to be 52 cycles per mm. When the image movement 
is added, the resolution drops to 34 cycles per mm., while for 
an initial modulation of 0.2 the resolution is still lower, at 
24 cycles per mm. 
The image movement MTF actually includes alternating neg 
ative and positive lobes at frequencies higher than the first 
zero, but these are at low amplitude and are not usually signif 
icant. Thus the first lobe has 22% modulation and would fall 
just on the threshold line for a high contrast target but well 
below it for a low contrast target.