Each Roll Film Magazine incorporates a Film Speed Dial (ISO settings 25 to 6400) which automatically interfaces through the AE Prism Finder with the camera shutter and lens diaphragm to assure correct exposures. Set it when you load the film. Other desirable Magazine fail-safe features are: Its Dark Slide is locked when Roll Film Magazine is not attached to camera to prevent accidental exposure; The shutter cannot be released when a Magazine is attached to camera, unless Dark Slide is withdrawn; The Magazines cannot be removed from camera unless Dark Slide is inserted. Convenient, built-in darkslide storage slot. 120 and 220 Magazines accept either 120 or 220 Inserts.