PITTSBURGH—A federal judge rejected a request for leniency in sentencing a western Pennsylvania man to eight years in prison for possessing child pornography, saying the defendant was "not simply a viewer" of such images.

Instead, federal prosecutors say 51-year-old William Blumentritt, of Jeannette, was fixated on father-daughter sex and tried to encourage someone he thought was the mother of a 9-year-old girl during online chats to subject the child to abuses. The "mother" was really an FBI agent with whom Blumentritt traded some images online in 2009.

Assistant public defender Thomas Livingston argued for a mandatory minimum sentence of just five years in prison, adding Blumentritt would be barred from using computers thereafter.

But Senior U.S. District Judge Gustave Diamond refused Tuesday saying Blumentritt "was not simply a viewer" of child porn images, but actively traded them online.