Archaeology Underwater: The NAS Guide to Principles and Practice

PHOTOGRAPHY 79

taken with a digital camera), and stitched together in the
digital darkroom. The alternative is to use a digital video
camera, with a diver or remotely operated vehicle (ROV)
swimming over the site at a fixed distance, making a
sequence of passes. The video camera could also be
mounted on a neutrally buoyant underwater sled pushed
along by a diver. Images are then captured, perhaps one
per second, and stitched together in a similar fashion. The
final mosaic can be adjusted to produce a balanced image
in terms of contrast, brightness, and colour. Harsh lines
and the like can be removed by using image-processing
methods like cloning. However, it should be noted that
mosaics only work well on flat sites.
There are various methods of collecting still images for
a photomosaic: for example, setting up a lightweight
photo-tower and systematically moving it along the
wreck, at a pre-set distance from the subject (plate 10.1),
taking photographs with a 50 per cent overlap. Alter-
natively, a diver or ROV can swim along a pre-set tape
measure or acoustic survey line or track. Much of the
success of the final mosaic is dependent on the care used
in collecting the images (figures 10.7 and 10.8).
In the past, the Nikonos system has most commonly
been used, often with a 15 mm wide-angle lens. However,
the 20 mm lens is more suited as a compromise between
optical distortion inherent in wider-angled lenses and
the desirability of keeping the camera-to-subject distance
short enough to ensure good image clarity. Only the

central part of the image is used (the perimeter being dis-

carded) in order to reduce optical distortion. This would

similarly apply to the use of a digital video camera. A

number of ‘panorama packages’ are currently available and

the technique is described in greater detail in ‘An under-

water photo-mosaic technique using Adobe Photoshop’

by Colin and Edward Martin (Martin and Martin, 2002).

Video Cameras

Video footage can serve as a useful extension to still

photography, especially under water (figure 10.9). It can

be used to demonstrate techniques, assist with survey

and to produce mosaics or a short documentary about the

project. Too many projects have a video archive that

never gets touched and the information never used or

disseminated.

Video is only useful if it is properly thought out and

edited. Once the purpose of the video has been established,

then a rough script can be worked out and a shot-list

of both surface and underwater images produced. One

method is to produce a storyboard, which is a series

of sketches visualizing each of the arranged shots. From

this, a shooting plan can be devised and the images

taken. These are then edited, which involves download-

ing them onto a computer and joining them together,

allowing further refinement. The edited video can then

be dubbed with a voice-over and/or background music.

Titles and credits can be added to the introduction

Figure 10.7 A photographic tower positioned on a rigid
site grid. (Drawing by Graham Scott)

D = distance of coverage required (m)

H = tower height (m)

W = width of film (mm)

f = focal length of the lens (mm) multiplied

by 1.33 for use under water

W f H D D H

W

=f

Figure 10.8 Photomosaics: formula to calculate lens focal

length and camera height necessary to give the required

coverage. (After Green, 2004:171)