The double mask system allows you to combine two scenes in one the effect is achieved by exposing each frame twice one with each filter contained in the double mask set one filter covers the center of the image the other the surrounding frame the system comes in two sizes 342 and 345 which has a larger center spot this double exposure can be done either by re cocking the camera without advancing the film or by exposing the whole roll of film with the first filter and then rewinding the