228 Forensic dentistrydeveloped achromatic lenses, which act to bring two different wavelengths to
a single coincident focus, or apochromatic, which bring together three colors
to a single focal point, many readily available chromatic lenses may require a
focus shift for nonvisible light wavelengths. There are several published opin-
ions concerning the correction of the focal point for nonvisible light ultra-
violet photography. Kodak^19 has suggested a simple one, and this is the one
the authors recommend you try first. It is Kodak’s opinion that the focus
shift required for ultraviolet photographs may be accounted for by simply
increasing the depth of field.^29 The recommendation is to decrease the lens
aperture at least two stops if shooting from wide open. Since the construction
of compound lenses can be so different, Kodak suggests that test exposures
at various aperture settings be performed to determine the exact change for
an individual lens.18,33 Cutignola and Bullough19,28 support Kodak’s recom-
mendation in correcting the ultraviolet focus by changing the depth of field.^
The downside to this modification is that it may significantly alter exposure
times, lighting, and film speed.
Other authors have suggested small focus shifts by turning the focus-
ing ring slightly from the visible focus position. Arnold et al.,^20 Lunnon,^21
Nieuwenhuis,^22 and Williams and Williams^23 have suggested shifting focus
for UV photography in the same direction and amount as is done in infrared
photography. The majority of modern high-quality achromatic compound
lenses have a focus color correction to achieve sharp photos.
The authors of this chapter have found that no focus shift is required
for ultraviolet photographs when using lenses designed specifically for UV,
such as lenses constructed of silicon-based (quartz) glass. Exposures using a
silicon lens have produced very sharp ultraviolet photographs with no shift
from the visible focus (Figure 11.30).
As forensic photography evolves, manufacturers are continuously modi-
fying and upgrading equipment. For this reason, it is difficult to recommend
any particular brand of equipment. There are digital cameras specifically
designed for nonvisible light photography. Each application for recording the
nonvisible ends of the light spectrum requires specific filters that allow only
that portion of the spectrum to pass through the lens.
Finding the optimal camera setup, the correct focal point, and a depend-
able source of lighting takes some research and many sessions of experi-
mental trials. The photographer should exercise patience and remember to
record the exposures and f-stops with every trial photograph taken in order
to determine the optimal parameters.
When doing infrared photography, a focus shift is required. This focus
shift moves the focus point of the object being photographed away from the vis-
ible focus since the actual infrared focus in patterned injuries in skin is below
the surface of the skin.^31 Many commercially available lenses have a small
mark on the focus ring that represents the infrared focus (Figure 11.31).