48
focal length in relation to the small image area (24 x 36 mm). The lens is of
fixfocus-type which is another advantage from the measuring point of view.
The two cameras are rigidly mounted on an aluminium L-profile, that fits
on standard photo tripods. The negative scale is 1:6, the base is 0.7 m and the
angle of convergence of the cameras 30 g . More detailed information is given
in Table 12.
Fig. 12.
Stereo-Medical-Nikkor, convergent stereocamera. The view-finders are used to
project two crosses on the object. When they coincide on the object this indicates
the best focussing distance and 100 % stereoscopic overlap.
8.2.2 Test Object
The method of obtaining a space grid is described in [30]. In this case the
three-dimensional test object was produced in the following way. The auto
graph Wild A6 has a drawing table, that can be moved vertically. The move
ment is recorded on accurate glass-scales. A light box with a glass grid was pla
ced on this drawing table. With spirit levels and theodolites the glass grid was
adjusted to be perpendicular to the movement. By exposing a negative three
times with the glass grid in three different positions we obtained a space grid.
A large number of photographed points (60) in the space grid were measured
for the purpose of calibrating the camera. Most of the calculations were done
on a CD 3600 computer.