Experiments have been carried out to study the field electron emission characteristics of nitrogen-doped amorphous diamond thin films prepared by magnetic-field filtered carbon ion deposition. A transparent anode imaging technique is used to record the spatial distribution of individual emission sites and the total emission current-voltage characteristics of the films. Also, the optical and electrical properties of the films having different nitrogen-doping levels have been studied. A correlation has been found to exist between the field-emission characteristics and the band gap of the films; i.e., it is found that the films of relatively small optical band gap have low turn-on fields.