Digital photographic and video evidence can be powerful stuff - and that's why it had better be authentic. From a technical perspective, there are two key points: 1) Obtain the original image, and 2) demand production of all metadata attached to every digital image file.
As experts know, most digital images degrade every time they are saved - to crop, resize, or reformat, for example. But with the original in hand, a forensic analyst is able to examine the maximum amount of pixel data, which makes possible the best analysis of the image in question.
Metadata is equally important, for it reveals the history of the digital file, including whether an image has been altered and to what extent. Metadata helps explain the version of the image being offered into evidence, and it's also good fodder for cross examination if an attorney needs to attack the authenticity or evidentiary value of that image.
Digital photos and videos are often saved in a format that reduces the image quality. Moreover, they degrade further and lose critical detail. The loss of quality usually occurs in color values and fine details of light and shadow - important components of the image itself. In addition, multiple re-saves and high rates of compression (often done to reduce the file size) can result in a distortion of the shapes of objects. And if the image has been printed, that process means even more degradation.
Aside from such incidental degradation, there is the possibility that someone has deliberately tampered with the evidence. For example, suppose there is a digital photograph of an accident scene, taken just after an automobile collision. It shows the intersection, two mangled cars, but no people. Has a bystander been "Photoshopped" out? Having the original image could help locate a vital eyewitness to the accident.
Metadata is information about the digital file - separate from the image itself. This can include, among other particulars, the camera make, model, and serial number, and the date and time the file was created or modified. Metadata can also indicate the order in which a series of photographs was taken, and whether the camera lens caused distortion. It may further indicate if software was used to save the image, and sometimes includes a history log of specific image edits, which can point to image alteration.
If photo or video imagery will ultimately be used at trial, it must be authenticated. (See California Evidence Code §§ 250, 1401(a).) In most cases, this is a simple matter of the photographer, or some other knowledgeable witness, stating that the image represents what he or she actually saw.
But how can an image that is not a camera original represent what it purports to show? It's possible. If the image was merely resaved (without any changes) using software such as Adobe Photoshop or Google's Picasa, there may be no problem. But the only way to know for sure whether an image has been altered significantly is to compare what has been offered with the original image, or to have it analyzed.
A forensic image analyst can often determine if an image has been altered. This involves a review of pixel dimensions, properties of the embedded thumbnail and preview images, the quantization tables (JPEG compression formulas), and related information.
To see a true camera original - in its unaltered state - one needs to see either the actual original file or a bit-for-bit copy.
What to Request
Pretrial discovery procedures grant litigants the right to obtain and review the best evidence possible, and for digital photos and videos this means looking at the earliest existing versions of every image. In some cases, this will require retaining a forensic computer expert to access hard drives and camera memory cards to recover deleted files. In cases where a court order is needed to force compliance with a discovery request, a declaration from a qualified expert about what files or metadata are missing can help make a persuasive showing to the judge.
The most reliable way to ensure that digital photo evidence is authentic is to request production of the original - and if there is any reason to believe the image doesn't show what it purports to, have it analyzed by a savvy digital expert.
George Reis owns Imaging Forensics in Orange County, providing forensic photographic and video analysis, and photography services.