Only warrants that "explicitly specify the area to be searched and the person or items to be seized" may be obtained, according to the Fourth Amendment. Is there a prohibition on an officer collecting materials thought to be of evidentiary value if they are discovered during the collection procedure specified in the warrant?

If it is not prohibited, are there rules/criteria that must be followed to assure the admissibility of the evidence?

How does this affect how digital material is "searched"?

For instance, suppose the examiner is seeking proof of identity theft and stumbles across a.jpg file containing child pornography.

Can the examiner check for more instances of child pornography without obtaining a new warrant or extending the present warrant?