15 - 
between a laboratory test and practical performance. 
Nothing of this sort is obtainable directly from the 
MTF. It might be considered that such information is 
unnecessary, and that the MTF being a test of the lens 
alone, should stand on its own. This might have some 
plausibility if all MTF's had the same shape. The MTF's 
of photographic lenses in general fall very rapidly from 
zero frequency to some low value such as 0.2 or 0.3, and 
continue at a very low level, say 0.1 or less, to the 
cut-off frequency for the aperture. Depending on the 
state of correction, different MTF's may cross at various 
levels. When such crossing occurs, how shall the lenses 
be evaluated? Some weighting technique, taking account 
of the emulsion and the human observer is clearly required. 
It has been suggested (4,5) that since each emulsion has a 
"threshold", indicating the minimum modulation it can 
detectably record at any spatial frequency, the area bet 
ween the MTF and the threshold would be a figure of merit 
for the lens in combination with that emulsion. This is 
not a rigorously proved relationship, but might be a useful 
practical guide. Thus in figure 1 it is not clear from 
the MTF's alone if lens A is superior or inferior to B. 
But the effective area of A is relatively greater than B for 
use with Tri-X emulsion, while without reference to the 
emulsion, B might be considered superior because its MTF 
continues to higher frequencies at a much higher modulation 
than A. Experience shows that lens A would give much better 
image quality in general aerial photography and the illus 
tration emphasizes that a lens cannot be evaluated solely 
from its MTF without reference to an emulsion. A fairly 
realistic appraisal of the relative importance of different 
parts of the MTF can be obtained if it is multiplied by the 
MTF of the emulsion with which the lens will be used. Diff 
erences between lenses are then reduced in apparent import 
ance and regions of very low MTF may appear to become in 
significant. 
Since the MTF is in frequency space a figure of merit 
might appear to be obtainable from the efficiency with which 
it passes the spatial frequency spectrum of "the typical 
scene" at image scale. However, there would be great difficulty 
in agreeing on a typical scene and measuring its spectrum, and 
the requirement for including the emulsion and observer would 
still have to be observed. 
The most practical approach to this problem involves 
the translation of the MTF into a graph of contrast versus 
size for some specified form of target. There are some advant 
ages in using a single bar target for this purpose. (6) The 
size/contrast graph provides information in a readily compre 
hended form. In conjunction with empirical data for minimum