CLOSE-UP STEREOPHOTOGRAPHY
USING A SINGLE CAMERA
Gunnar Simonsson
Department of Photogrammetry, Stockholm
Stereoscopic photography of fixed objects
using a single camera can be performed in
accordance with any, for instance the normal,
case by moving the camera from the one base
point to the other between the two exposures.
The method has long been used for instance
with photo-theodolites. For the stereoscopic
viewing of the photographs there are no
stringent requirements placed on the relative
orientation of the camera. For photogrammet-
ric measurements, however, it is necessary
to know or to be able to reconstruct the
relative orientation.
In comparator photogrammetry combined with
data processing the relative as well as the
absolute orientation can be calculated using
y-parallaxes and control point co-ordinates.
This method is excellent for the numerical
determination of points but is not practical
for graphical evaluation (for instance the
drawing of contour lines and sections) due
to the great demand it places on equipment
and soft-ware. Therefore, in production
graphical work is usually performed in an
analogue instrument, such as the wild A7.
To achieve efficiency and good economy in
graphical photogrammetry using analogue
instruments one fundamental requirement is
that the stereo-camera (or the device holding
the single-camera) should be stable enough
to guarantee the same relative orientation
from the one stereo-pair to the other.
In order to make the fulfilment of this re-
quirement meaningful the two photographs must
be adjusted very accurately in the photograph
holders of the instrument, with respect to
the inner orientation.
To achieve this accurate inner orientation of
the photographs, a special photograph adjust-
ing device is needed, this device being placed
in the photograph holder of the stereo instru-
ment.
With the conditions above fulfilled the one
Stereo-pair after the other can be oriented
.in the instrument with practically no change
in the orientation elements. Practical expe-
rience has shown that the only adjustment
needed is a slight change of elements to ob-
tain correct distances in the direction of
the camera axis. This "convergence adjustment"
can be done either by adjusting $ of one of
the projectors or by moving the photograph
in the one photograph holder in the x-direc-
tion using the photograph adjusting device.
Which method should be used cannot be stated
generally, and is not of great practical im-
portance as the adjustment needed is very
small.
To accomplish the "convergence adjustment" no
known distances are needed if the camera base
is constant and a bar with two marks, distant
from each other by the length of the base, is
placed parallel to the base and photographed
at the same time as the object. The discre-
pancy in distance in the direction of the
camera axis is eliminated with the base of the
stereo instrument set at zero.
Any photograph from a camera with a stable
inner orientation can be adjusted in the
photograph holder with an accuracy sufficient
for most practical purposes, even with no or
defective fiducial marks. With the first
Stereo-pair oriented in the instrument the
co-ordinates of the fiducial marks (or any
identification marks or patterns in the photo-
graphs) are recorded and used for the adjust-
ment of the succeeding stereo-pairs with the
aid of the photograph adjusting device. If
the photographs are oriented in k as a first
step only one fiducial mark is needed in each
photograph to reconstruct its position in the
photograph holder.
DESCRIPTION OF THE DEVICE FOR
CLOSE-UP STEREOPHOTOGRAPHY
WITH A SINGLE CAMERA
Single camera stereophotographs with a good
stability with respect to relative orienta-
tion can be taken at the Department of Photo-
grammetry with a camera mounted in a camera
holder designed and constructed according to
the principles shown in Figure 1.
The camera is placed unsymmetrically on a
frame with four legs (C-Al, C-A2, D-Bl, D-B2
in Figure l1). The legs fit accurately into
four holes in a stable base, as is illustra-
ted by the dashed arrows. For one of the
camera positions the legs C-Al and C-A2 are
fitted into the holes 1 and 2. For the other
camera position the legs D-Bl and D-B2 are
fitted into the same holes. (The other camera
position is dashed in the figure). The photo-
graphy base can be varied. It is double the
distance from the camera to the center of the
camera holder. A flash lamp or a photo-flood
lamp may be placed in the center of the hol-
der as shown in Figure 1.
The camera axis will be reconstructed with a
high precision from one exposure to the other,
firstly, because of the great distance between
the holes and, secondly because of the nature
of the framework of the holder, which, assu-
ming the base frame is stable, is itself a
stable structure. This further means that
only slender members are required for the
frame which theoretically need only be pin-
