" 
distinct vision, where the natural base-to-height ratio of approximately 1:4 is the 
“natural” maximum, i.e. where the disparity, or parallax, is one quarter of the height 
producing it. At greater distances these are less, at lesser distances everything is 
indistinct. 
| If we ignore meaningless ideas and diagrams regarding depth perception based on 
"convergence", as we must in cases of "divergence" or even for “parallel” or “distant” 
vision, we will realize that we see depth by disparity in the field surrounding the point 
of attention, and that it means nothing to our sense of depth that we may have to 
exercise some eye muscles in order to look at the same "point of attention" with both 
eyes, as when attempting to "fuse" an inappropriately mounted stereogram. 
If we have learned to recognize the appearance of a cube best when disparity equals 
one quarter the height—or the height is 4 times the disparity—we will best "recognize" 
the height of a tree or car or building in a stereo pair under similar circumstances, 
which only exist when H — 4B or f = 4b, 
where H is the flying height 
B is the air base 
f is the focal length 
b is the photo base. 
The degree of rer apa 1 : p b 1 
1e degree of exaggeration can then be expressed as the ratio of or to -, or 
H 4 
E ; 4B 4b 
“xaggeration —— = 
DE H f. 
In the case of "normal" photography, f — 6 in. format 9 X 9 and 6007, overlap, 
P. : : 4b 4. x 3.0 : 
b = 3.6 in. and Exaggeration = => 2 = 2.4x 
cc [. — ) 
For no exaggeration, b , and in the case quoted b = 1.5 in. which is 17% of the 
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picture width and the overlap is 839%. A compromise of 80% is suggested for good 
interpretability since it also provides the “normal” 609; overlap with alternate photos. 
Larger scales give finer detail—until image motion intervenes, or excessive base-to- 
height ratios (due to minimum camera cycling time) produce gross exaggeration of 
height and only permit “examination by layers”, with much double imagery and little 
perception of the original as a whole. There is no satisfaction in climbing trees 
stereoscopically! 
( Examples of forest photography at different scales and overlap are to be included 
in the slides in stereo-projection.) 
The Positive 
On rare occasions it may be feasible and advantageous to use the negatives directly 
for measurement, or for other forms of interpretation. This eliminates the printing 
process but involves cutting the roll film and using a light table (for inspection under 
a stereoscope). It also introduces the risk of damage to the only record—the original 
negative. In most cases a positive of some form will be required. 
The difficulty of reproducing in the positive all of the information recorded in 
the negative is well known. The problem is aggravated by a long density range in the 
o 
negative (due either to a long brightness range in the subject or to over-development), 
and by the short available tone range in printing paper, particularly when examined, 
as usually, by reflected light. Conversely, it is somewhat alleviated with negatives of 
shorter density range, or by making diapositives (and prints, on suitable paper) for 
inspection by “transmitted light, and it can be practically eliminated by using one of 
( 
c 
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