A device for detecting perforations in film includes a camera body or other optical instrument body. A pressure plate is disposed at a rear of the body and has a channel that allows access of a sensor output to detect and read film position via the recognition of such perforations in the moving film as it passes through the camera or optical instrument body. The pressure plate maintains the alignment of the film relative to the body. An installation board is aligned and mounted adjacent a rear of the pressure plate. A sensor is attached to the installation board. The sensor is disposed to face a section of the film having perforations through the channel of the pressure plate as the film moves along a film path. The sensor transmits electronic signals corresponding to the position of perforations of the film as the film moves along the film path. The invention provides for the efficient and economical construction of a device having a sensor for detecting perforations in film.