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.