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For crime scene investigators, processing a scene entails recognizing evidence, note taking, scene photography, measurements, evidence collection and submission. It also includes searching for and developing impression evidence, such as fingerprints, palm prints, footprints, tool marks, footwear impressions, tire tracks and other forms of impressions. Not every impression can be collected in place, lifted or cast. While all evidence requires photographic documentation prior to recovery, in many instances photography is the only way to recover the evidence itself. Anyone who has used photography to recover impressions knows the process is much more involved than simply taking a picture. In this article, I’ll discuss some of the keys to photographing impression evidence. 

Owen McDonnell

Camera and Image Resolution. In the past, the primary crime scene camera was a 35 mm (36 mm x 24 mm negative) SLR (single lens reflex), and obtaining comparison-quality images with sufficient resolution was usually not a problem. But with the transition to digital photography, image resolution can be a concern, especially with older model cameras that may not provide the necessary resolution. For instance, the Scientific Working Group on Image Technology mandates a minimum resolution of 1000 pixels per inch (ppi) for friction skin images, which means a single fingerprint block of 1" x 1.5" requires 1.5 MP (megapixels) to capture an image at 1000 ppi. Larger impressions, such as a palm print of 4" x 4", require 16 MP. For other evidence, resolution is mandated, but 500 ppi is considered the minimum for comparison purposes. Note that to capture an 8" x 12" segment of a tire impression at 500 ppi, your camera must be capable of capturing a 24 MP image. Many newer DSLRs are capable of 24 MP and 36 MP. Most current cell phone cameras can also render an image of sufficient quality provided you can limit the image to the impression being captured. Remember to consider the capabilities of your camera before attempting to capture impression evidence.

Camera Lens. What’s the smallest area the lens can capture? To find out, I recommend using an ABFO or Footwear/Tire track L scale at the lens’ closest focus setting. Then gradually move the camera back from the scale until it’s in focus. This technique is known as pre-focusing. During this test, once the marking on the scale is in focus, you’ll know the closest you can be to an object and obtain a focus, and the smallest area your lens is capable of capturing. If you need to capture smaller areas, adding a 1:1 or 1:2 macro lens to your photography kit will allow you to easily obtain great images. If cost is an issue, you can use extension tubes or a set of screw-on diopters. Camera lens diopters are sold in groups of three (+1, +2 and +4) and can be stacked to obtain the required magnification. 

Scales. Every photograph for comparison must include a scale. Why is this so important? To answer this question, consider two images of the same brand and model of a shoe. In each image, the shoe occupies the full frame of the camera. One image could be a child’s shoe and the other a size 14 men’s shoe. Without a scale, it’s almost impossible to determine which one is which. Including the scale allows the image to be calibrated in image processing programs. The two calibrated images can then be compared in detail at the same scale. In addition, the scale must be on the same plane as the impressions for an accurate calibration. A slight variance in height between the impression and the scale will result in a variance during calibration. To bring the scale and the impression into alignment may require raising the scale or lowering it to the surface. To ensure someone doesn’t claim your scale is covering something up, always take a photograph prior to adding the scale. 

Aperture. For overall mid-range and close-up photography, we’ve all been taught to use an aperture of at least f/8 to ensure good depth of field. However, in impression photography, depth of field is not usually a concern, so you can use larger f/stops. Of course, bracketing your f/stop by +1 and -1 is always best practice. 
Digital Capture Sensor. For best results, the sensor must be parallel to the surface the impression rests on. Sounds complicated; it’s not really, as the sensor lies parallel to the back of the camera. If the back of the camera is parallel to the impression, then the sensor is too.

Lighting. When using a flash, you must take it off the hot shoe. For a flash to work correctly, there must be sufficient distance for the light to strike the surface and be reflected to the center of the lens. When photographing impressions such as tool marks or friction skin impressions, you’ll be working at relatively close distances, which means there won’t be enough distance between the flash head and the lens. One option for dealing with this problem is to use a ring light or other flash device that attaches to the lens, allowing illumination. Ring lights do a great job when photographing impression evidence, such as powder-developed latent impressions and some tool marks. In addition, when photographing most impressions, including plastic, dust or other impressed images, oblique or side lighting provides much better detail than direct lighting. Vary the angle of light to achieve maximum contrast, including shadows to increase visible details. You can also use a flashlight to achieve a low oblique angle and to fan over the impression. Another option is a PC cord or remote flash trigger that allows the flash to be placed at angles to achieve optimal lighting. You may need a bounce card on the opposite side of the impression to allow the light to partially fill in the shadows created during oblique lighting.

Stability. While you may be able to hand hold the camera to photograph impressions in some instances, in other cases, doing so causes shaking and blurred images. Placing the camera on a tripod or a Quadrapod and using a remote shutter will provide greater stability and result in much better results. Locking the camera in place will also allow you to position lighting for maximum results. 
A tripod is even more critical when photographing footwear or tire track impressions. Most tripods allow the camera mount tube to be inverted so that the camera can be positioned upside down between the legs of the tripod. Each impression or section (in tire tracks) requires 10 photographs. Take one photograph prior to the addition of any scale or marker. Then take a series of three photographs with the flash fired at an angle between each set of the tripod legs. Each series consists of bracketed (-1,0,+1) apertures. In order to ensure adequate image resolution, no more than 12" to 14" should be captured in a single photograph. For longer impressions, photograph in segments with at least a 1" overlap from the previous segment. This technique allows the images to be stitched together accurately. Since a tire impression is rarely less than 12" long, overlapping segments of the impression will be required. One rotation of a 205/65R15 tire will require approximately 80.16" (2035 mm) of impression to be captured if available.

Photographing impression evidence can be challenging. Using proper equipment, honing your skills in advance, and taking your time at the scene will allow you to capture impression evidence that may have gone uncollected otherwise.

Owen McDonnell retired as the Lieutenant/Supervisor of the Caddo Sheriff’s Office Crime Scene Investigations Division in Shreveport, LA after 31 years. He is the owner of M.O. Forensics LLC and provides consulting and training in crime scene and fingerprint development and comparison techniques, as well as heading workshops through IAI. He holds IAI certifications as a Senior Crime Scene Analyst, Ten Print Fingerprint Examiner and Latent Print Examiner. McDonnell holds a Master of Forensic Science Administration Degree from Oklahoma State University Center for Health Sciences.

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