In the U.S. Federal Rules of Evidence, the entry for Rule 1002 reads as follows:
"To prove the content of a writing,
recording, or photograph, the original
writing, recording, or photograph is
required, except as otherwise provided
in these rules or by Act of Congress."
The best evidence rule calls for the use of primary (original) documents whenever possible. This also applies to images, whether these are photographic, x-ray, digital, audio files or otherwise. However, federal rules and California state law allow the use of mechanical reproductions (such as photocopies) unless there are valid reasons to suspect their authenticity, or if the party to a court action can demonstrate that its use would compromise fairness in the proceeding.
The best evidence rule dates back well over 260 years; in those days, legal documents were usually created by hand, despite the availability of the printing press. These copies were usually made by a clerk, but could also be made by one of the litigants themselves. Legal experts at the time felt that copies were likely to contain errors that could affect the proceedings. Therefore, it was preferable to produce the original document whenever possible.
The digital age has produced an entire new set of problems that threaten to render the best evidence rules obsolete. Often, when a document has been digitized, it is difficult to determine which one is the original, since some processes produce multiple electronic duplicates all at once. Unfortunately, there are many ways to alter digital images using applications such as Photoshop, and it is often impossible to tell if and how an image has been altered when the process is done by an expert working with high-end computer equipment.
This rule of evidence is also known as the "original writing rule," despite the fact that it can be applied to a number of different types of documents.