Imaging Forensics: So, What's the Story on Resolution?

My post on Synchronizing Video has generated a few comments related to image resolution. Three people, all of whom have a significant background in forensic imaging, have made rather divergent comments. Multiple topics are being addressed, from crime scene, accident and evidence photography to latent print photography to SWGFAST guidelines/requirements to the resolution of video security camera systems.

In all of these, there is a common element regarding resolution - the image should have the resolution to see the details for the purpose of the analysis. In a crime scene, this may be enough resolution to see the relationship of different objects in the image to each other. In a fingerprint, this may be second or third level detail. In the security camera it may be enough to read a license plate or see individualizing features in an object or person.

In security video, I almost always wish there was more detail, or higher resolution. These images are frequently 640 X 480 pixels or less. In addition, they are often highly compressed, the camera is at a bad angle, the lighting is poor, the optics are poor quality and dirty, and the subject of interest is frequently in the distance. Very few in the forensic video analysis field would argue that we'd all prefer security video with more resolution.

In fingerprints, I have seen some pretty good prints at resolutions as low as 300 PPI. The SWGFAST guidelines currently call for a minimum 1,000 PPI at a native resolution. So, can a fingerprint examiner use images that are less than 1,000 PPI or that may have been scanned with a scanner that interpolates when set to 1000 PPI? Of course they can - and they do.

Regarding crime scene, accident and evidence photography, many people compare digital imaging to film and suggest only using digital cameras that match the resolution of film. When I ask these proponents which film format (disc, 110, APS, 35mm, 6 X 6, 4 X 5), which film brand, which ISO, using which optics, whether they are concerned with tonal resolution as well as optical resolution, etc., they usually are unable to answer. There is nothing special about film that makes it a standard for measuring resolution.

When taking photographs, we simply need to know what level of detail is required, then be certain that we capture that level of detail. Arbitrary numbers, like the SWGFAST 1,000 PPI help us to guarantee that we are capturing enough detail, but we also need to understand that resolutions less than those recommendations don't somehow become useless.