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Proper photography techniques are essential to the documentation and analysis of impression evidence.Proper photography techniques are essential to the documentation and analysis of impression evidence.

Following a few simple guidelines for close up impression evidence photography helps ensure sufficient detail is recorded, which in turn enables the examiner to conduct a complete and thorough comparison.

In general, crime scene track evidence documentation should include the basic series of photographs advised for all crime scene evidence: overall photographs, midrange photographs, and close-up or examination quality photographs.

Overall photographs include the tracks or impressions and relate their location within the general context of the crime scene. Midrange photographs relate the impressions to their specific location within the crime scene and to other nearby items of evidence, including other impressions. Overall and mid range photographs are important to document the position and orientation of impression evidence; however, they are not considered suitable for a complete forensic comparison.With the exception of being able to eliminate a shoe or tire based on an outsole or tire tread design that may be visible in an overall or midrange photograph, to conduct a complete comparison a high quality, properly composed close-up photograph of the impression should be taken.

For the photography of impression evidence, it is recommended that a professional SLR (single lens reflex) camera is used. A minimum of 8 megapixel resolution is recommended by the Scientific Working Group on Imaging Technology (SWGIT). Interchangeable lenses, detachable flash (with a 6 foot cord or remote capability), manual control of exposure, and remote shutter release are also recommended by SWGIT and by the Scientific Working Group for Shoeprint and Tire Tread Evidence (SWGTREAD).

A close-up photograph, or examination quality photograph, of a footwear or tire tread impression should be taken utilizing proper camera position, composition, scale, identifier, lighting, and be of sufficient resolution. Each of these aspects allows the most accurate and detailed natural size enlargement to be produced for the examiner to utilize in the comparison process.

To position a camera for impression evidence photography, place it on a tripod, position the focal plane parallel to the impression, and ensure the camera is at a sufficient distance from the impression to avoid using a wide angle.Wide angle views can create distortion around the edges of the photograph. Not placing the focal plane parallel to the impression can also cause a visual change in the apparent width or length of the impression depending upon the angle from which the photo is taken. The use of a tripod allows for stability while taking the photograph, preventing quality problems that arise from shaking while hand holding a camera during longer exposures.

The photograph should be composed to include the entire footwear impression, or in the case of tires, approximately a 12" segment of the track. Both the scale and the identifier (evidence item number) should also be visible in the photograph. A small Post-it®, piece of tape, or paper used to include the identifier on the scale is recommended. Using a larger tent style photo marker in the close up requires a larger area to be photographed, and less detail is captured. The impression, scale, and identifier should fill the frame of the photograph. The scale is positioned close to the impression, but not on top of any of the detail of the impression to avoid damage of the impression. For this reason, scales should not be placed across the tread of a tire impression.

Figure 1: Proper camera position for taking an examination quality photograph.

For footwear impressions, an L shaped scale (also called a Bureau scale) is recommended. It possesses all the desirable qualities of a good impression scale: it is rigid, finely divided, flat, non-reflective, and will photograph clearly in a variety of lighting conditions. These scales are often reverse color, one side primarily black and the other side primarily white. In general, the side of the scale is chosen to match the color of the background. For instance the white side of the scale may be used for photographing impressions in snow or light colored sand. The dark side of the scale might be used to photograph an impression in dark mud or in the laboratory for photographing impressions on black gelatin lifts. The 90 degree angle of the L scale will appear true in photographs taken from the proper angle, with the focal plane parallel to the impression. If the angle of the camera is other than parallel, then the scale will appear to have other than a right angle. Some three-dimensional impressions make it difficult to properly place an L shaped scale, in these instances a straight ruler with similar qualities (rigid, finely divided, flat, non reflective) is used.

To conduct a comparison, an actual size photographic enlargement is made using the scale in the crime scene photograph. For a two-dimensional photograph, such as an impression in blood on a tile floor, the scale is appropriately placed on the floor next to the impression. In the case of a three-dimensional impression, the goal is to place the scale at the same plane as the bottom of the impression. When this is accomplished, and the scale is used to enlarge the impression, the actual size of the impression of the outsole of the shoe is enlarged to natural size. If the scale sits above the impression, on the top of the snow or mud that the impression is made in, and the enlargement is made using the scale, then the actual impression of the outsole will be smaller in the enlarged photograph than it actually is. Since the examiner knows this in general, but does not know how much smaller, this limits the comparison in terms of an evaluation to the size of a given shoe.

Figure 2: Scales should be placed at the same level as the bottom of the impression.

Have you ever wondered why in fishing magazines the person holding the catch thrusts the fish toward the camera? A change of only a few inches depth in the photograph can make a 15 lb. fish look like a 20 lb one! Take a look at the photos in Figure 3, which halibut looks better? Same fish, one is just one leg closer to the camera! This illustrates the effect of small differences in height between a scale and an outsole impression.

Figure 3: These images illustrate the importance of placing the scale at the correct depth relative to an impression.

Examination quality photographs of tire impressions are taken with the same attributes as footwear impressions, except the photographs are taken as an overlapping series. Along ruler is extended next to and along the tire track. A series of photographs are taken of approximately 12" widths, filling each frame with a segment of the tire impression and a section of the long ruler. The next and subsequent photographs are taken, moving along the track, overlapping approximately 3" for each photograph in the field of view.An identifier (not a photo tent, as previously described) relating to the track is placed in each frame. The ruler is not moved during the series of photographs. When the series of photographs are enlarged to actual size by an examiner, they are pieced together to form an actual size photographic replication of the track for comparison to a tire.

Contrast is another important aspect of the quality of the recorded impression. To increase contrast in the photograph, both lighting techniques and some spray materials can be useful. Lighting from the side of the impression (oblique light) creates shadows that highlight the design detail. The optimum angle of light varies with the depth of three-dimensional impressions. Oblique light may also improve the visibility of detail in some two-dimensional impressions. It is helpful to use a flashlight to determine the best angle light to use to photograph an impression. It is most effective to look through the camera viewfinder during this determination, because the lighting that looks best from this vantage point will also look best when recorded by the camera.

Once the best angle of light is determined, at least three photographs should be taken with light coming from three different directions, approximately even distances apart around the impression. One method is to shine the light from between each pair of tripod legs. The varying views are to ensure that detail in the shadow in one impression will be visible and highlighted in one or more of the other photographs. To control the lighting it may be necessary to shade the impression, either with a blanket or another object that can provide a shadow. In snow, highlighting the impression with gray primer or snow print wax to increase the contrast in the photograph usually works better than oblique light. A word of caution; spraying snow with a dark coating material may cause the impression to melt quickly in ambient sunlight. Melting time is a consideration when allotting time for photography and casting of an impression in snow.

If any enhancement methods are applied to a footwear or tire impression, photograph the impression before and after each process. This is to ensure the detail at each step is captured and recorded. It is recommended that impressions be cast, lifted, or enhanced depending on their nature, in addition to a photograph of the original impression as found.

Figure 4: Oblique lighting enhances contrast.

The examiner using your photograph to conduct a comparison can do their best job if they can be assured of the shape and size of the footwear or tire track impression. This is accomplished by using the proper camera position, scale, and positioning of the scale or tape. Techniques and equipment that provide high quality detail of outsole and tire tread design information, as well as record fine detail such as wear and damage, allow the examiner to conduct comparisons of these characteristics. In the discipline of footwear and tire track evidence, the more accurate and detailed the photographs are the more complete and conclusive the comparison conducted can be.

Resources

  • Bodziak, W. Footwear Impression Evidence 2nd ed.; CRC Press: Florida 2000.
  • Robinson, E. Crime Scene Photography 2nd ed.; Elsevier/Academic Press 2010.
  • Scientific Working Group on Footwear and Tire Track Evidence Guide for the Documentation and Photography of Footwear and Tire Impressions at the Crime Scene http://www.swgtread.org/standards/published.html accessed April 14, 2011.
  • Scientific Working Group on Imaging Technology General Guidelines for Photographing Footwear Impressions and General Guidelines for Photographing Tire Impressions http://www.theiai.org/guidelines/swgit/index.php accessed April 14, 2011.

Lesley Hammer is a forensic footwear, tire track, and crime scene consultant and instructor and is the current chair of SWGTREAD. Lesley can be reached at hammer.forensics@gmail.com.

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