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.