8 REPORT OF COMMISSION VII
plotted image contrast as a function of focal setting with detail size as a param-
eter. Macdonald found that, as detail size increases, there is a shift to the
right of the focal setting which gives optimum tone contrast. For most types of
photo-interpretation, we are as interested in one detail size as we are in another,
so that it is difficult to exploit this discovery. In certain special types of inter-
pretation, however, we may be primarily interested in objects of relatively uni-
form size, in which case proper adjustment of the camera's focal setting might
yield superior photography for interpretation purposes. The following are repre-
sentative examples of these special types as applied to vertical aerial photo-
graphic interpretation: (1) The making of traffic surveys in which automobiles of
relatively uniform size are to be detected and enumerated; (2) the making of
personnel counts at a public gathering, or of troops on a battlefield; (3) the mak-
ing of a fish or wildlife census from photography on which the particular popula-
tion to be detected and enumerated usually is remarkably uniform in size; (4)
the making of /og counts in a sawmill pond or free counts and related measure-
ments in an even-aged stand of timber.
The possible improvement in photographic image quality resulting from em-
ployment of any one of the foregoing special techniques, admittedly may be
rather slight. But in some cases a relatively slight improvement may result in
much more useful photography, as evidenced by the excellent work of Black-
well (1946) and Macdonald (1951, 1952). For example, the work of these in-
vestigators, relative to the visual contrast threshold of the human eye, shows
clearly that a relatively small increase in tone contrast (e.g., through proper
choice of film and filter, or through improved processing methods, or through
employment of some of the other techniques mentioned above) many greatly
increase the probability of detection.
The cumulative effect that might be achieved from the simultaneous em-
ployment of several of the special techniques described in this section might be
very appreciable indeed. This matter would seem to constitute a very fruitful
field for further research.
PHOTOGRAPHIC INTERPRETATION EQUIPMENT, MATERIALS AND METHODS
Of the many links which constitute the photo-reconnaissance, photo-inter-
pretation chain, the weakest ones are believed by many to fall under this second
broad heading. Certain recently developed types of photogrammetric equip-
ment such as the Kelsh plotter, the Kail plotter, the McNeil Oblique Plotter, the
Stereosimplex, Twinplex and the Wild A-6 plotter may faciliate photo-interpreta-
tion either directly or indirectly, but much can yet be done in the perfection
of equipment, materials and methods
primarily for use by the photo-inter-
preter.
A. STEREOSCOPES FOR FIELD USE:
At least two types of lens stereo-
scopes have been developed in recent
years which are well suited to field
use. The first of these is exemplified
by the Ryker stereoscope, illustrated
in Figure 3 and the second by the
U.S.G.S. type. Both types provide a
F1G. 3. An example of a lens type of stereo- ;
scope well suited to field use. (Courtesy of Harri- firm but portable base upon which
son C. Ryker, Inc.) the stereo pair of photos can be
