All digital evidentiary photographs must be captured in a color mode, even if the object photographed is black and white or the final output will be grayscale. This is true regardless of whether the impression evidence is scanned using a flatbed scanner or photographed using a flatbed scanner. More specifically, photographing forensic evidence in color lets you capture subtle tones that can be lost when photographing or scanning in grayscale mode. During image processing, the digital image will be converted into grayscale.
In addition, you must photograph all forensic evidence using a camera RAW format with the highest bit-depth (such as 12-bit or 14-bit color depth). During image processing, the camera RAW data are converted and the processed image is saved as a TIF file.
Digital photographing forensic evidence requires the perfect trifecta: ISO, shutter speed, and aperture. As I mentioned earlier, “Auto” is not an acceptable camera setting for photographing forensic evidence of any kind: latent prints, footwear, tire tread, trace evidence, etc. I strongly suggest examiners set the camera to fully manual mode using the following guidelines.
In a lab with bright lights on the copy stand, I recommend using an aperture setting of f/16, an ISO of 100, and a shutter speed of 1/125.
Again, depending upon the type of lights affixed to the copy stand, it may be necessary to “shed more light” on the evidence (allow more light). For less brighter lights, I recommend stepping down the aperture to f/11 (or even f/8 depending upon the “sweet spot” of the lens.
To help determine what aperture, ISO and shutter speed to use, I typically tell new crime lab personnel to set a coffee cup on the copy stand and then turn on the lights on the copy stand and look at the shadow of the coffee cup. They should also check evaluate the shadow thrown by the coffee cup with just available light (no lights on the copy stand) or with their forensic light source.
The following table will provide a quick reference for “starter” settings. The aperture or shutter speed may need to be adjusted to allow more or less light.
while an overcast day would require f/5.6. Normally, changing the f-stop would require you to also change your shutter speed to ensure an even exposure. With the Sunny 16 rule, disregard that. Simply remember to keep you shutter speed set to the film speed ISO and the aperture to the amount of sun available.
Here is a quick table to help you understand the Sunny 16 rule, how it applies to different film speeds, and how different amounts of sun will affect the f-stop.
Barely Visible Shadow
Shutter Speed for ISO 100
Shutter Speed for ISO 200
Shutter Speed for ISO 400
Shutter Speed for ISO 800
When photographing latent impressions developed using a chemical or dye stain, you are photographing the luminescence or fluorescence of the chemical’s reaction (reflection) with the impression, not the light (wavelength) generated by the forensic light source (laser or ALS).
In other words, after you process the evidence, and you must choose the correct barrier filter (red, orange, or yellow) that allows you to distinguish the contrast between the light reflected from the ridge detail and the light absorbed by the background.