PHOTOGRAPHING SPECIMENS
Because photogrammetry requires parallax
either the camera or the specimen must move
between shots, or both. Depending on the size
of the specimen and its mobility, as well as the
space available, one of two main methods is
usually adopted: the turntable method, or the
walk-around method. For the former the
camera is usually stationary, ideally on a
tripod. For the latter, the specimen usually
does not move and the camera is moved
around it, either hand-held or on a tripod. Both
methods have their specific advantages and
demands. Only occasionally is it advantageous
to move both the specimen and the camera, for
example when the turntable method is used
with a scale bar that cannot be placed on the
turntable with the specimen.
Aside from choosing between the methods the
image acquisition process is influenced also by
considerations regarding the potential risk of
failure of the in-program model creation.
Photography aimed at optimizing the work
process for the one-chunk model creation
method described below results in images that,
if the one-chunk method fails, do not make full
use of the superior image alignment capabilities
of the multi-chunk method. We therefore
recommend reading the sections Alignment
methods for more than one set of images
and Background – blank or structured?
beforehand.
The turntable method: basic principles
The turntable method has the camera on a
tripod, and a series of photographs is taken of
the specimen on a turntable that is rotated
across a small angle between shots. Figure 4
shows the typical process for photographing a
specimen on a turntable. The photograph series
thus forms a perfect circle of camera positions
around the specimen, with the camera always
aimed at the central vertical axis of the
turntable. In order to cover the surface of a
specimen with a complex geometry it is usually
necessary to vary the height of the camera
position in relation to the specimen by
repeating the process shown in Figure 4 with
the camera at a different height, so that
several concentric circles of photographs are
taken. Undercuts may require additional
photographs, with the camera pointed off the
main vertical axis. These images are usually
created not by moving the camera, but by
manually shifting the specimen on the turntable
and optimizing the position for each
photograph. The underside of the specimen has
to be photographed by flipping it over, and
sometimes more than two positions are
required to capture the full surface geometry.
The biggest advantages of the turntable
method are the ability to control the lighting,
photograph speedily, with fixed camera
settings, and the control one has over the
background. The non-turning background must
be as devoid of features as possible or masked,
so that no points are detected on it. Masking is
additional work and should thus be avoided,
but it can allow combining photograph series
from different specimen positions into one
photogrammetric reconstructions without any
need to adjust partial models to each other
(see one-chunk method below), delivering the
highest quality models for the least amount of
fiddling and editing.
The biggest drawback of the turntable method
is, aside from the requirement of a featureless
background, that it requires the specimens to
be mobile, and sturdy enough to be handled.
Additionally, even the sturdiest turntables have
weight limits, so that very large specimens like
sauropod dinosaur sacra or elephant skulls are
too heavy.