It was used while Peter Venkman was trying to communicate with the Library ghost. It is unclear if the Camera was successful in photographing her, however from this research done during the Library Ghost encounter helped them to figure out how to capture and store ghosts.
Ray used his camera to photograph Peter Venkman holding the newly discovered Poveglian Artifact. He sent copies of the photos to Egon Spengler for a translation of the faint Swedish writing under the artifact's design.
The Leather case (aka: Nikon FM2 FE2 FE Camera Leather Case) is also seen on Ray and is sometimes mistaken for another unknown prop.