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An interesting attachment for close-up stereo-photography is made by RBT. It
was designed expressly for
their own dual lens 35mm cameras, but the principle may be
adapted to twin camera stereo rigs as well. This macro
device nicely solves the problem of obtaining a stereo base
separation of several millimeters, while the two 35mm frames
recording the images are much wider, about 70mm apart, as
shown in the image of a normal RBT camera, below.
An RBT twin-lens stereo
camera consists of highly modified pair of normal SLR's,
joined at the hip so-to-speak. The
lens separation is too great for macro photography.
However, the lenses can be removed, and the macro attachment
shown below can be mounted in their place.
The
RBT macro stereo camera (using a different body than that
shown above). It employs a single lens to image the
subject from two slightly shifted perspectives. Behind the lens are
an aperture stop and
mirrors that direct the closely separated left and right beams
onto the two widely separated film chips.
If there is a fairly large
distance between the macro lens and the film plane, it is
possible to insert mirrors in this space in order to spread
the neighboring left and right
beams far enough to use individual full frames of 35mm film for
each side. Lenses appropriate for this sort of design
are typical 100mm focal length, or longer.
Sketch
of the light path looking down on the top of the RBT macro
attachment. A bellows allows changes in magnification
from 1:1 to 1:3. Focussing is acheived by changing the
camera-to-subject distance. Just behind the lens is a
plate with two small holes separated by either 6 or 11
mm. This gives the stereo base separation. The
holes are small, so a high f-number is attained, leading to
fairly substantial depth-of-field. Two
pairs of front surface mirrors direct the left and right beams
to their respective films via the lens mount openings on the
two-lens RBT camera. Such a device is sometimes called a
"beamspreader."
A
composite view of the lens opening, showing the two base
separations, along with a fully open position (for focusing).
The
front of the RBT macro attachment. Looking into the lens
one can see the separate aperture holes (yellowish circular
spots). This system is usually used with short-distance
electronic flash because at effective f-stops between 45 and
90 (approximately) substantial illumination is required.
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