4 
Angle of field 
The question of the most favourable angle of field was 
investigated in the course of extensive tests. With due 
regard to the camera-to-object distances and to the 
constant base length selected, and also in consideration 
of the chosen standard 6.5 cm x 9 cm plate size, the 
optimun focal length was found with 64 mm. 
With the choice of the long 80 mm format side horizontal 
positioned, the resulting horizontal field angle is 64° 
(71®). Compared to a longer focal length this means that 
objects of considerable extension in depth can be imaged 
stereoscopically without any necessity to increase the 
camera-to-object distance which would prejudice the econ 
omy and be unfavourabe for the accuracy of plotting. A 
greater depth range is desired, for instance, when the 
chalked skid marks of fairly long braking distances are 
to be photographed. Conversely a further reduction of 
focal length would entail too great a reduction of the 
picture scale and with it, reduced identifiability of 
essential details. A rather unnatural perspective, 
bothersome when viewing single pictures, would also be 
a consequence. The short vertical picture side was . 
limited to 60 mm, and the picture frame was shifted ver 
tically by 10 mm with respect to the principal horizontal 
plane. For the normal position of the base tube, this 
results in the following angular field in the object 
space: 
an angle of depression of 32° (26.6S), plus 
an angle of elevation of 17°20' (19.3®) 
This displacement of the frame of the principal horizontal 
was introduced because of the greater field angle of 
the new lens and since it permitted omission of a base 
tube tilt. 
Experience shows that in short range photogrammetry, 
tilted photographs are not favoured. Besides, in forensic 
applications the objects are practically always on the 
ground. Thus the upper portion of the picture (in object 
space) is of no importance - a rather larger angular 
field below the horizontal is more useful. 
Omission of the tilting feature of the base tube might 
bring disadvantages for other applications however, such 
as for photograms of buildings. To prevent this, the 
base tube can be turned upside down and clamped to the 
elevator column platform; the greater field angle is 
then directed upward in the object space. 
This inversion feature endows the C120 Stereometric 
Camera with a significant versatility while maintaining 
the advantageous restriction to the simplest photo 
grammetric imaging disposition.