Abstract: Means and a method are disclosed by which a conventional camera can be converted from its usual full-frame format to a half-frame format. The conversion means comprises a mask removably locatable within the camera body directly forward (i.e. facing the lens rear surface) of the film in the camera, and having a window therein centrally located with respect to that aperture in the camera body defining the full-frame image format. The mask window is properly sized to provide a half-frame image format, effecting a 50% reduction in the exposed image area, by blocking quarter frame areas of the camera body aperture at either end thereof. The mask is preferably bowed rearwardly against the film's emulsion surface, so as to bias the film against the camera pressure plate.